[SR-4165] Add an UpToOne Collection

[dabrahams]

Previous ID	SR-4165
Radar	None
Original Reporter	@dabrahams
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Bug
Assignee	None
Priority	Medium

md5: f39656b2b2aeb1cf4e0042dba951599f

Issue Description:

If we had UpToOne<T> as an adapter over T? we could stop generating such a messy type from

(0..<10).lazy.flatMap { $0 < 5 ? nil : $0 }

It's currently LazyMapBidirectionalCollection<LazyFilterBidirectionalCollection<LazyMapBidirectionalCollection<(CountableRange<Int>), Int?>>, Int>

but could be

LazyCollection<FlattenBidirectionalCollection<LazyMapBidirectionalCollection<(CountableRange<Int>), UpToOne<Int>>>>

but more importantly, it would allow us to supply a single generic typealias for the result of calling lazy flatMaps (on bidirectional collections today; on all collections once we have conditional conformance).

[belkadan]

What's the criterion for inclusion for new helper types like this?

[dabrahams]

That they pass an -evolution review, innit?
We do have EmptyCollection and CollectionOfOne already; this is in the same ballpark, FWIW.