[Stdlib] Rationalize the SubSequence type of a filtered Collection.

[DougGregor]

Address ABI FIXMEs #28, #80, #81 by making the SubSequence of a filtered
collection into be a filtered collection of the base's
SubSequence rather than a slice of this filtered collection, e.g.,
replace Slice<LazyFilterCollection<Array<T>>> with
LazyFilterCollection<ArraySlice<T>>, which allows slicing
optimizations of the base type to kick in.

While here, eliminate the vestigial type LazyFilterIndex, which was
necessary pre-Swift-3 to allow the index to move. Swift 3's indexing
model means that the movement of indices is on the collection itself,
so we no longer need LazyFilterIndex: instead, the Index type of
the lazy filtered collection is simply the Index type of the base
collection, which is a nice convenience: it means you can take indices
from a lazy wrapper around a given collection C and use them with the
collection C (and, with care, vice-versa) without jumping through
extra hoops.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[DougGregor]

Another one for @dabrahams and @airspeedswift

[DougGregor]

@swift-ci please smoke test and merge

[DougGregor]

Hmm... I'm nervous about this one. The extra requirement on LazyFilter{whatever collection}

where Base.SubSequence: ${collectionForTraversal(Traversal)}

is going to break things in generic contexts. I don't think I can make this change in isolation; I think it has to follow recursive protocol constraints. Let's try to stop this merge :)

[DougGregor]

I'll still eliminate LazyFilterIndex, which was an independent change anyway.