Add new nil encoding methods to containers
Add new nil encoding methods to containers

* Add encodeNil(forKey:)/encodeNil() for keyed/unkeyed encoding
  containers
* Add decodeNil(forKey:)/decodeNil() for keyed/unkeyed decoding
  containers
* Give default implementation of decodeIfPresent(forKey:)/
  decodeIfPresent(_:) on keyed/unkeyed decoding containers instead of
  decode(forKey:)/decode(_:)
* Expose all encode/decode methods on keyed encoding & decoding
  containers to allow overriding default methods (which were previously
  not forwarding)

amraboelela/swift,ahoppen/swift,roambotics/swift,harlanhaskins/swift,roambotics/swift,glessard/swift,huonw/swift,swiftix/swift,felix91gr/swift,airspeedswift/swift,nathawes/swift,airspeedswift/swift,CodaFi/swift,xedin/swift,xedin/swift,brentdax/swift,swiftix/swift,gregomni/swift,uasys/swift,zisko/swift,jopamer/swift,harlanhaskins/swift,shajrawi/swift,nathawes/swift,aschwaighofer/swift,ahoppen/swift,JGiola/swift,frootloops/swift,xwu/swift,gribozavr/swift,karwa/swift,xwu/swift,tkremenek/swift,JGiola/swift,shajrawi/swift,shahmishal/swift,harlanhaskins/swift,OscarSwanros/swift,jopamer/swift,djwbrown/swift,parkera/swift,austinzheng/swift,jckarter/swift,djwbrown/swift,xedin/swift,jmgc/swift,benlangmuir/swift,shajrawi/swift,xwu/swift,CodaFi/swift,shajrawi/swift,hooman/swift,frootloops/swift,shahmishal/swift,return/swift,jckarter/swift,swiftix/swift,airspeedswift/swift,jopamer/swift,austinzheng/swift,xwu/swift,brentdax/swift,tkremenek/swift,atrick/swift,allevato/swift,shahmishal/swift,JGiola/swift,brentdax/swift,allevato/swift,alblue/swift,jopamer/swift,deyton/swift,frootloops/swift,aschwaighofer/swift,aschwaighofer/swift,djwbrown/swift,sschiau/swift,karwa/swift,frootloops/swift,tjw/swift,jckarter/swift,atrick/swift,ahoppen/swift,karwa/swift,karwa/swift,JGiola/swift,nathawes/swift,xedin/swift,frootloops/swift,OscarSwanros/swift,parkera/swift,djwbrown/swift,tkremenek/swift,djwbrown/swift,brentdax/swift,rudkx/swift,CodaFi/swift,jmgc/swift,karwa/swift,danielmartin/swift,rudkx/swift,alblue/swift,gregomni/swift,huonw/swift,apple/swift,amraboelela/swift,shahmishal/swift,zisko/swift,milseman/swift,danielmartin/swift,milseman/swift,apple/swift,parkera/swift,austinzheng/swift,OscarSwanros/swift,zisko/swift,stephentyrone/swift,aschwaighofer/swift,practicalswift/swift,felix91gr/swift,natecook1000/swift,jmgc/swift,huonw/swift,danielmartin/swift,jopamer/swift,jmgc/swift,airspeedswift/swift,jmgc/swift,allevato/swift,stephentyrone/swift,apple/swift,felix91gr/swift,lorentey/swift,uasys/swift,CodaFi/swift,jckarter/swift,practicalswift/swift,practicalswift/swift,uasys/swift,amraboelela/swift,practicalswift/swift,lorentey/swift,practicalswift/swift,shahmishal/swift,deyton/swift,CodaFi/swift,zisko/swift,swiftix/swift,lorentey/swift,atrick/swift,sschiau/swift,deyton/swift,benlangmuir/swift,practicalswift/swift,benlangmuir/swift,return/swift,karwa/swift,benlangmuir/swift,shahmishal/swift,glessard/swift,frootloops/swift,milseman/swift,nathawes/swift,roambotics/swift,airspeedswift/swift,glessard/swift,shahmishal/swift,karwa/swift,apple/swift,deyton/swift,zisko/swift,airspeedswift/swift,apple/swift,hooman/swift,brentdax/swift,return/swift,harlanhaskins/swift,JGiola/swift,jmgc/swift,shajrawi/swift,shahmishal/swift,alblue/swift,lorentey/swift,stephentyrone/swift,shajrawi/swift,stephentyrone/swift,uasys/swift,aschwaighofer/swift,jmgc/swift,stephentyrone/swift,roambotics/swift,jopamer/swift,tjw/swift,gribozavr/swift,allevato/swift,devincoughlin/swift,swiftix/swift,rudkx/swift,return/swift,deyton/swift,natecook1000/swift,swiftix/swift,gregomni/swift,hooman/swift,tjw/swift,practicalswift/swift,felix91gr/swift,gribozavr/swift,huonw/swift,milseman/swift,gribozavr/swift,rudkx/swift,frootloops/swift,ahoppen/swift,tkremenek/swift,xedin/swift,deyton/swift,gribozavr/swift,glessard/swift,tjw/swift,tkremenek/swift,OscarSwanros/swift,milseman/swift,harlanhaskins/swift,brentdax/swift,OscarSwanros/swift,airspeedswift/swift,parkera/swift,atrick/swift,return/swift,zisko/swift,austinzheng/swift,return/swift,natecook1000/swift,rudkx/swift,return/swift,xwu/swift,devincoughlin/swift,aschwaighofer/swift,benlangmuir/swift,xwu/swift,allevato/swift,uasys/swift,sschiau/swift,lorentey/swift,parkera/swift,aschwaighofer/swift,danielmartin/swift,xedin/swift,hooman/swift,OscarSwanros/swift,gribozavr/swift,alblue/swift,glessard/swift,parkera/swift,allevato/swift,danielmartin/swift,natecook1000/swift,amraboelela/swift,djwbrown/swift,jckarter/swift,gribozavr/swift,shajrawi/swift,apple/swift,alblue/swift,gribozavr/swift,OscarSwanros/swift,JGiola/swift,felix91gr/swift,djwbrown/swift,devincoughlin/swift,sschiau/swift,hooman/swift,sschiau/swift,devincoughlin/swift,xwu/swift,tjw/swift,milseman/swift,parkera/swift,austinzheng/swift,jopamer/swift,rudkx/swift,deyton/swift,ahoppen/swift,alblue/swift,brentdax/swift,jckarter/swift,alblue/swift,milseman/swift,tkremenek/swift,stephentyrone/swift,lorentey/swift,devincoughlin/swift,stephentyrone/swift,devincoughlin/swift,amraboelela/swift,huonw/swift,allevato/swift,tkremenek/swift,jckarter/swift,danielmartin/swift,felix91gr/swift,austinzheng/swift,amraboelela/swift,ahoppen/swift,nathawes/swift,CodaFi/swift,benlangmuir/swift,austinzheng/swift,zisko/swift,harlanhaskins/swift,gregomni/swift,natecook1000/swift,devincoughlin/swift,shajrawi/swift,roambotics/swift,tjw/swift,devincoughlin/swift,sschiau/swift,karwa/swift,nathawes/swift,gregomni/swift,parkera/swift,amraboelela/swift,glessard/swift,huonw/swift,gregomni/swift,hooman/swift,huonw/swift,sschiau/swift,xedin/swift,hooman/swift,lorentey/swift,nathawes/swift,uasys/swift,sschiau/swift,lorentey/swift,uasys/swift,atrick/swift,harlanhaskins/swift,natecook1000/swift,natecook1000/swift,danielmartin/swift,practicalswift/swift,swiftix/swift,roambotics/swift,xedin/swift,felix91gr/swift,CodaFi/swift,atrick/swift,tjw/swift