[docs] Start of Modules User Model: the high-level purpose and semantics.
[docs] Start of Modules User Model: the high-level purpose and semantics.

Still to come:
- submodules
- source files as modules
- import search paths
- ObjC interop stuff

For a later doc:
- build model

Things to note as future work:
- namespaces
- fragility
- SPI vs. API

Swift SVN r8745

CodaFi/swift,calebd/swift,ben-ng/swift,gmilos/swift,apple/swift,shahmishal/swift,mightydeveloper/swift,stephentyrone/swift,lorentey/swift,JaSpa/swift,parkera/swift,tjw/swift,karwa/swift,ahoppen/swift,jtbandes/swift,rudkx/swift,SwiftAndroid/swift,kperryua/swift,hooman/swift,IngmarStein/swift,swiftix/swift.old,deyton/swift,Jnosh/swift,brentdax/swift,practicalswift/swift,hughbe/swift,ken0nek/swift,zisko/swift,CodaFi/swift,codestergit/swift,tjw/swift,milseman/swift,apple/swift,devincoughlin/swift,gribozavr/swift,IngmarStein/swift,jopamer/swift,kusl/swift,swiftix/swift.old,Jnosh/swift,swiftix/swift,adrfer/swift,allevato/swift,kperryua/swift,Jnosh/swift,swiftix/swift.old,ben-ng/swift,harlanhaskins/swift,modocache/swift,xwu/swift,nathawes/swift,emilstahl/swift,khizkhiz/swift,apple/swift,bitjammer/swift,jopamer/swift,bitjammer/swift,jtbandes/swift,SwiftAndroid/swift,CodaFi/swift,CodaFi/swift,gregomni/swift,MukeshKumarS/Swift,brentdax/swift,danielmartin/swift,jckarter/swift,jtbandes/swift,frootloops/swift,OscarSwanros/swift,devincoughlin/swift,kusl/swift,jmgc/swift,hooman/swift,tardieu/swift,xedin/swift,khizkhiz/swift,tinysun212/swift-windows,glessard/swift,codestergit/swift,xwu/swift,LeoShimonaka/swift,brentdax/swift,cbrentharris/swift,uasys/swift,xedin/swift,jckarter/swift,Jnosh/swift,IngmarStein/swift,rudkx/swift,hughbe/swift,natecook1000/swift,slavapestov/swift,return/swift,austinzheng/swift,deyton/swift,xwu/swift,milseman/swift,codestergit/swift,tjw/swift,parkera/swift,uasys/swift,tinysun212/swift-windows,austinzheng/swift,sschiau/swift,swiftix/swift.old,kperryua/swift,natecook1000/swift,kentya6/swift,swiftix/swift.old,dduan/swift,shajrawi/swift,Jnosh/swift,dreamsxin/swift,hughbe/swift,jckarter/swift,calebd/swift,tardieu/swift,karwa/swift,nathawes/swift,gribozavr/swift,SwiftAndroid/swift,return/swift,lorentey/swift,gmilos/swift,shahmishal/swift,airspeedswift/swift,IngmarStein/swift,xwu/swift,kstaring/swift,adrfer/swift,benlangmuir/swift,stephentyrone/swift,tjw/swift,alblue/swift,deyton/swift,JaSpa/swift,return/swift,lorentey/swift,glessard/swift,gmilos/swift,sdulal/swift,JaSpa/swift,mightydeveloper/swift,LeoShimonaka/swift,xedin/swift,sschiau/swift,kentya6/swift,huonw/swift,swiftix/swift,cbrentharris/swift,manavgabhawala/swift,frootloops/swift,LeoShimonaka/swift,practicalswift/swift,airspeedswift/swift,rudkx/swift,danielmartin/swift,devincoughlin/swift,roambotics/swift,glessard/swift,tardieu/swift,kstaring/swift,roambotics/swift,felix91gr/swift,JaSpa/swift,tjw/swift,Ivacker/swift,sdulal/swift,rudkx/swift,shahmishal/swift,mightydeveloper/swift,huonw/swift,uasys/swift,karwa/swift,harlanhaskins/swift,harlanhaskins/swift,swiftix/swift,gottesmm/swift,kstaring/swift,gottesmm/swift,gottesmm/swift,cbrentharris/swift,Ivacker/swift,amraboelela/swift,emilstahl/swift,lorentey/swift,apple/swift,therealbnut/swift,aschwaighofer/swift,LeoShimonaka/swift,IngmarStein/swift,emilstahl/swift,russbishop/swift,modocache/swift,aschwaighofer/swift,kentya6/swift,rudkx/swift,LeoShimonaka/swift,KrishMunot/swift,therealbnut/swift,sdulal/swift,manavgabhawala/swift,johnno1962d/swift,practicalswift/swift,KrishMunot/swift,milseman/swift,therealbnut/swift,tjw/swift,JGiola/swift,KrishMunot/swift,shahmishal/swift,xedin/swift,dduan/swift,cbrentharris/swift,arvedviehweger/swift,devincoughlin/swift,xedin/swift,adrfer/swift,tardieu/swift,MukeshKumarS/Swift,rudkx/swift,modocache/swift,emilstahl/swift,therealbnut/swift,airspeedswift/swift,austinzheng/swift,JaSpa/swift,felix91gr/swift,codestergit/swift,lorentey/swift,kentya6/swift,alblue/swift,xwu/swift,therealbnut/swift,stephentyrone/swift,kperryua/swift,lorentey/swift,parkera/swift,glessard/swift,parkera/swift,khizkhiz/swift,JGiola/swift,danielmartin/swift,apple/swift,karwa/swift,allevato/swift,karwa/swift,amraboelela/swift,Jnosh/swift,benlangmuir/swift,manavgabhawala/swift,practicalswift/swift,xwu/swift,return/swift,KrishMunot/swift,swiftix/swift.old,atrick/swift,uasys/swift,jtbandes/swift,MukeshKumarS/Swift,MukeshKumarS/Swift,sschiau/swift,OscarSwanros/swift,arvedviehweger/swift,aschwaighofer/swift,gribozavr/swift,parkera/swift,JaSpa/swift,harlanhaskins/swift,gribozavr/swift,JGiola/swift,codestergit/swift,slavapestov/swift,uasys/swift,aschwaighofer/swift,brentdax/swift,ken0nek/swift,manavgabhawala/swift,hughbe/swift,Ivacker/swift,felix91gr/swift,frootloops/swift,frootloops/swift,brentdax/swift,OscarSwanros/swift,xedin/swift,arvedviehweger/swift,benlangmuir/swift,jmgc/swift,stephentyrone/swift,karwa/swift,alblue/swift,airspeedswift/swift,atrick/swift,gmilos/swift,nathawes/swift,jmgc/swift,sschiau/swift,austinzheng/swift,xwu/swift,atrick/swift,cbrentharris/swift,amraboelela/swift,tinysun212/swift-windows,djwbrown/swift,sdulal/swift,dduan/swift,natecook1000/swift,practicalswift/swift,SwiftAndroid/swift,sdulal/swift,KrishMunot/swift,uasys/swift,hooman/swift,slavapestov/swift,tjw/swift,stephentyrone/swift,benlangmuir/swift,djwbrown/swift,sschiau/swift,johnno1962d/swift,adrfer/swift,OscarSwanros/swift,xedin/swift,russbishop/swift,zisko/swift,alblue/swift,austinzheng/swift,modocache/swift,jmgc/swift,karwa/swift,huonw/swift,parkera/swift,shajrawi/swift,cbrentharris/swift,parkera/swift,zisko/swift,Ivacker/swift,ben-ng/swift,harlanhaskins/swift,russbishop/swift,therealbnut/swift,OscarSwanros/swift,frootloops/swift,roambotics/swift,gribozavr/swift,shahmishal/swift,sschiau/swift,ken0nek/swift,natecook1000/swift,danielmartin/swift,aschwaighofer/swift,emilstahl/swift,modocache/swift,arvedviehweger/swift,practicalswift/swift,russbishop/swift,amraboelela/swift,natecook1000/swift,codestergit/swift,ahoppen/swift,aschwaighofer/swift,shajrawi/swift,nathawes/swift,danielmartin/swift,jmgc/swift,deyton/swift,arvedviehweger/swift,cbrentharris/swift,glessard/swift,benlangmuir/swift,emilstahl/swift,sdulal/swift,hughbe/swift,jopamer/swift,tkremenek/swift,airspeedswift/swift,shajrawi/swift,allevato/swift,return/swift,gottesmm/swift,benlangmuir/swift,calebd/swift,deyton/swift,MukeshKumarS/Swift,nathawes/swift,hooman/swift,kstaring/swift,LeoShimonaka/swift,bitjammer/swift,harlanhaskins/swift,tinysun212/swift-windows,kusl/swift,gmilos/swift,mightydeveloper/swift,JGiola/swift,frootloops/swift,harlanhaskins/swift,tardieu/swift,dduan/swift,russbishop/swift,slavapestov/swift,gregomni/swift,zisko/swift,swiftix/swift,SwiftAndroid/swift,tkremenek/swift,jckarter/swift,brentdax/swift,jopamer/swift,MukeshKumarS/Swift,gribozavr/swift,felix91gr/swift,JaSpa/swift,emilstahl/swift,huonw/swift,therealbnut/swift,manavgabhawala/swift,OscarSwanros/swift,shajrawi/swift,practicalswift/swift,swiftix/swift,arvedviehweger/swift,felix91gr/swift,swiftix/swift,devincoughlin/swift,sschiau/swift,tkremenek/swift,xedin/swift,bitjammer/swift,johnno1962d/swift,ahoppen/swift,jmgc/swift,shahmishal/swift,stephentyrone/swift,kperryua/swift,jopamer/swift,gottesmm/swift,mightydeveloper/swift,johnno1962d/swift,airspeedswift/swift,atrick/swift,calebd/swift,slavapestov/swift,jmgc/swift,roambotics/swift,ken0nek/swift,russbishop/swift,SwiftAndroid/swift,tinysun212/swift-windows,allevato/swift,Ivacker/swift,aschwaighofer/swift,ahoppen/swift,dduan/swift,allevato/swift,jckarter/swift,khizkhiz/swift,milseman/swift,kperryua/swift,tkremenek/swift,kstaring/swift,KrishMunot/swift,zisko/swift,djwbrown/swift,devincoughlin/swift,JGiola/swift,kentya6/swift,IngmarStein/swift,alblue/swift,jckarter/swift,ahoppen/swift,djwbrown/swift,apple/swift,JGiola/swift,hooman/swift,gribozavr/swift,LeoShimonaka/swift,swiftix/swift,parkera/swift,kstaring/swift,kstaring/swift,Jnosh/swift,swiftix/swift.old,khizkhiz/swift,gregomni/swift,huonw/swift,gregomni/swift,adrfer/swift,jtbandes/swift,milseman/swift,IngmarStein/swift,dreamsxin/swift,calebd/swift,jopamer/swift,roambotics/swift,milseman/swift,tkremenek/swift,djwbrown/swift,kentya6/swift,return/swift,nathawes/swift,arvedviehweger/swift,allevato/swift,gribozavr/swift,roambotics/swift,frootloops/swift,modocache/swift,devincoughlin/swift,Ivacker/swift,jopamer/swift,gmilos/swift,danielmartin/swift,swiftix/swift.old,hooman/swift,djwbrown/swift,tkremenek/swift,bitjammer/swift,stephentyrone/swift,modocache/swift,alblue/swift,natecook1000/swift,devincoughlin/swift,alblue/swift,brentdax/swift,tardieu/swift,austinzheng/swift,practicalswift/swift,shahmishal/swift,mightydeveloper/swift,atrick/swift,kperryua/swift,cbrentharris/swift,sschiau/swift,kusl/swift,sdulal/swift,kentya6/swift,lorentey/swift,johnno1962d/swift,dduan/swift,mightydeveloper/swift,ken0nek/swift,zisko/swift,jtbandes/swift,bitjammer/swift,karwa/swift,kusl/swift,kusl/swift,hughbe/swift,CodaFi/swift,bitjammer/swift,johnno1962d/swift,khizkhiz/swift,mightydeveloper/swift,johnno1962d/swift,gmilos/swift,deyton/swift,gottesmm/swift,shajrawi/swift,calebd/swift,atrick/swift,amraboelela/swift,sdulal/swift,uasys/swift,amraboelela/swift,hughbe/swift,gregomni/swift,gregomni/swift,LeoShimonaka/swift,tinysun212/swift-windows,Ivacker/swift,ben-ng/swift,airspeedswift/swift,danielmartin/swift,MukeshKumarS/Swift,calebd/swift,Ivacker/swift,adrfer/swift,return/swift,russbishop/swift,manavgabhawala/swift,ken0nek/swift,CodaFi/swift,huonw/swift,amraboelela/swift,austinzheng/swift,codestergit/swift,allevato/swift,tinysun212/swift-windows,lorentey/swift,ken0nek/swift,emilstahl/swift,ben-ng/swift,gottesmm/swift,shajrawi/swift,slavapestov/swift,milseman/swift,khizkhiz/swift,tkremenek/swift,slavapestov/swift,ahoppen/swift,huonw/swift,kusl/swift,shahmishal/swift,glessard/swift,hooman/swift,OscarSwanros/swift,ben-ng/swift,natecook1000/swift,deyton/swift,felix91gr/swift,manavgabhawala/swift,felix91gr/swift,jtbandes/swift,tardieu/swift,SwiftAndroid/swift,nathawes/swift,djwbrown/swift,kusl/swift,adrfer/swift,ben-ng/swift,zisko/swift,kentya6/swift,dduan/swift,jckarter/swift,CodaFi/swift,KrishMunot/swift,shajrawi/swift