Add per thread allocation counters, and enhance heap sampling.

Add the "thread.allocated" and "thread.deallocated" mallctls, which can
be used to query the total number of bytes ever allocated/deallocated by
the calling thread.

Add s2u() and sa2u(), which can be used to compute the usable size that
will result from an allocation request of a particular size/alignment.

Re-factor ipalloc() to use sa2u().

Enhance the heap profiler to trigger samples based on usable size,
rather than request size.  This has a subtle, but important, impact on
the accuracy of heap sampling.  For example, previous to this change,
16- and 17-byte objects were sampled at nearly the same rate, but
17-byte objects actually consume 32 bytes each.  Therefore it was
possible for the sample to be somewhat skewed compared to actual memory
usage of the allocated objects.