Attempt to implement a real AsyncSequence

Currently running into a compiler crash.
Merge branch 'master' into async-sequence
Merge branch 'master' into async-sequence

Conflicts:
	RxSwift/AsyncSequence.swift
	RxSwift/Observable.swift
Document and standardize AsyncSequence style
Move _FlattenScanGenerator to top-level

Works around the compiler crash.
Merge branch 'master' into async-sequence

Conflicts:
	RxSwift.xcodeproj/project.pbxproj
	RxSwift/Observable.swift
Fix AsyncSequence.empty definition
Add AsyncSequence.error
Add AsyncSequence.concat
Add AsyncSequence.materialize
Add AsyncSequence.zipWith
Merge pull request #4 from jspahrsummers/async-sequence

AsyncSequence and AsyncBuffer