Addressed ABI FIXME’s #4, apple#5, apple#104 and apple#105, making Sequence’s
SubSequence conform to Sequence, with the same element type, and for
which the SubSequence of a SubSequence is the same SubSequence.

Fixes SR-318 / rdar://problem/31418206.

…ection.

Introduce (recursive) constraints that make the *Collection constraint
of SubSequence match that of its enclosing *Collection, e.g.,
MutableCollection.SubSequence conforms to MutableCollection.

Fixes rdar://problem/20715031 and more of SR-3453.

Make the Indices types conform to the appropriate Collection protocol:
* Collection.Indices: Collection
* BidirectionalCollection.Indices: BidirectionalCollection
* RandomAccessCollection.Indices: RandomAccessCollection

…traints.

Eliminates a few explicit uses of the Indexable protocols.

Rather than using the default slice type when slicing the collection produced
by a lazy map or filter, slice the base collection and form a new
lazy map/filter collection from it. This allows any optimizations provided by
the collection SubSequence type to kick in, as well as ensuring that slicing
a lazy collection provides the same type as producing a lazy collection of a
slice.

This is technically source-breaking, because someone could have spelled out
the types of slicing a lazy filter or map… but it seems unlikely to matter
in practice and the benefits could be significant.

Fixes ABI FIXME’s apple#28 and apple#46.

The various _*Indexable protocols only exist to work around the lack of
recursive protocol constraints. Eliminate all of the *_Indexable protocols,
collapsing their requirements into the corresponding Collection protocol
(e.g., _MutableIndexable —> Collection).

This introduces a number of extraneous requirements into the various
Collection protocols to work around bugs in associated type
inference. Specifically, to work around the lack of "global" inference
of associated type witnesses. These hacks were implicitly present in
the *Indexable protocols; I've made marked them as ABI FIXMEs here so
we can remove them when associated type inference improves.

Fixes rdar://problem/21935030 and a number of ABI FIXMEs in the library.