[RFA 0/5] remove cleanups from regcache

[Tom Tromey <tom@tromey.com>]

This series removes the remaining cleanups from regcache and related
code.

First, it removes regcache_xmalloc and regcache_xfree in favor of
using plain new and delete everywhere.  These were already used under
the hood; this just removes a layer of indirection.

Second, the main patch, #3, removes a cleanup by using
std::unique_ptr.

The final two patches remove some cleanups locally in regcache.c by
using standard techniques.

Regression tested by the buildbot.

Tom