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.
