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.
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