[SR-474] Implement SE-0014: Constraining AnySequence.init

[DougGregor]

Previous ID	SR-474
Radar	None
Original Reporter	@DougGregor
Type	Bug
Status	Closed
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Bug
Assignee	@moiseev
Priority	Medium

md5: dcf1a7aa261ee6c36bd3765ed9c1ba86

Issue Description:

Implement SE-0014: Constraining AnySequence.init

https://github.com/apple/swift-evolution/blob/master/proposals/0014-constrained-AnySequence.md

[moiseev]

#895