swift234

stdlib: add MutableSlice

See doc comments on MutableSlice for more information about what it is.

MutableSlice was one of the reasons to clarify and tighten index
invalidation rules.  After that change, existing MinimalCollection
test types were performing checks that are too strict according to the
model.  Existing algorithms and collections could provide them, but not
MutableSlice.  This commit updates MinimalCollection types to perform
index invalidation checks that correspond to new rules.

Part of rdar://20722366.  This commit adds the type, but does not wire
it up completely yet.

Swift SVN r30839