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)

Author: Itai Ferber