[Proposal] introduce Dictionary.compactMapValues(_:) (#787)
[Proposal] introduce Dictionary.compactMapValues(_:) (#787)

* add 0000-introduce-compact-map-values

* edits/rewrite to clean up language issues

* fix typo

* make status awating review

* Number SE and schedule for review

anandabits/swift-evolution,rballard/swift-evolution,allevato/swift-evolution,abertelrud/swift-evolution,jckarter/swift-evolution,airspeedswift/swift-evolution,anandabits/swift-evolution,ddunbar/swift-evolution,DevAndArtist/swift-evolution,abertelrud/swift-evolution,rballard/swift-evolution,Nirma/swift-evolution,hartbit/swift-evolution,natecook1000/swift-evolution,Nirma/swift-evolution,apple/swift-evolution,jckarter/swift-evolution,DevAndArtist/swift-evolution,allevato/swift-evolution,airspeedswift/swift-evolution,apple/swift-evolution,hartbit/swift-evolution,ddunbar/swift-evolution,natecook1000/swift-evolution