[docs] Address some of Slava's feedback on LibraryEvolution.rst.
[docs] Address some of Slava's feedback on LibraryEvolution.rst.

(but not all of it)

- Add a note about throwing functions becoming non-throwing.
- Clarify that let-properties may not be turned into read-only vars.
- Permit new static stored properties on fixed_layout structs.
- Drop note about "fixed_layout-since-first-available".
- Add note disparaging the utility of @fixed.
- Add note about adding defaulted associated types to protcols.
- Clarify which members can and cannot be added to a class.
- Add some class-specific permitted changes.
- Note that adding/removing 'dynamic' is /not/ permitted.
- Split up protocol and non-protocol extensions.
- Explain why the 'no_payload' performance assertion isn't needed.

Saving the rest of the feedback for the next major content addition.

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