[stdlib] ExistentialCollection: Up/downgrades
[stdlib] ExistentialCollection: Up/downgrades

More-capable collections can be converted to less-capable.
Failable conversions are available in the opposite direction

Swift SVN r26306

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