Add support for the Hoard allocator.

Can't recommend that one use it though. On mat (our newish 24x Xeon
X5690 machine), on rw1, we see:

malloc: ~180K puts/sec
hoard: ~380K puts/sec
jemalloc: ~510K puts/sec
tcmalloc: ~580K puts/sec
flow: ~640K puts/sec