Add support for sized deallocation.

This adds a new `sdallocx` function to the external API, allowing the
size to be passed by the caller.  It avoids some extra reads in the
thread cache fast path.  In the case where stats are enabled, this
avoids the work of calculating the size from the pointer.

An assertion validates the size that's passed in, so enabling debugging
will allow users of the API to debug cases where an incorrect size is
passed in.

The performance win for a contrived microbenchmark doing an allocation
and immediately freeing it is ~10%.  It may have a different impact on a
real workload.

Closes #28

include/jemalloc/internal/private_symbols.txt

@@ -61,6 +61,7 @@ arena_ralloc_no_move
 arena_redzone_corruption
 arena_run_regind
 arena_salloc
+arena_sdalloc
 arena_stats_merge
 arena_tcache_fill_small
 arenas
@@ -228,7 +229,9 @@ iralloc
 iralloct
 iralloct_realign
 isalloc
+isdalloct
 isthreaded
+isqalloc
 ivsalloc
 ixalloc
 jemalloc_postfork_child