[SR-6864] Add a "cycle" method to the standard library

[airspeedswift]

Previous ID	SR-6864
Radar	None
Original Reporter	@airspeedswift
Type	New Feature

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	New Feature, StarterProposal
Assignee	None
Priority	Medium

md5: ce3f8a07c6a7edad8e1997fb1e825980

Issue Description:

It is often useful to cycle over a sequence indefinitely. For example, just like you can write zip(repeatElement("foo", count: Int.max), myArray) and zip(1..., myArray), you could write {{zip([.grey,.white].cycle(), myArray)}}.

[swift-ci]

Comment by erica sadun (JIRA)

What the hey.

Forum pitch: [[Starter Pitch] Introducing a `cycled` method to `Sequence`|https://forums.swift.org/t/starter-pitch-introducing-a-cycled-method-to-sequence/11254/1]

Gist: https://gist.github.com/erica/97e75cd0b6ce9908fea9a6ac0d83276e