swift234

Eliminate all of the uses of ++/-- from stdlib/public/core.

At DaveA's suggestion, I took a mostly mechanical approach to this:
pointers and numeric types start using += 1, and indexes use
i = i.successor().  The index model is likely to be revised in
Swift 3 anyway, so micro-optimizing this code syntactically isn't
super important.

There is some performance concern of this patch, since some
in-place succesor operations are more efficient than
i = i.successor().  The one that seems particularly at issue is the
instance in the implementation of partition(), which I changed to
use i._successorInPlace().  If other instances lead to a perf issue,
they can be changed to use that as well.