Add unsized and sized deallocation fastpaths. Similar to the malloc()
fastpath, this removes all frame manipulation for the majority of
free() calls. The performance advantages here are less than that
of the malloc() fastpath, but from prod tests seems to still be half
a percent or so of improvement.
Stats and sampling a both supported (sdallocx needs a sampling check,
for rtree lookups slab will only be set for unsampled objects).
We don't support flush, any flush requests go to the slowpath.
794e29c0ab