tmp
Memory pools

This patch implements the `flt_pool` memory pool interface.  It's very
similar to libcork's memory pool implementation, but is aware of fleet
execution contexts.  Each context maintains its own list of recycled
objects; `flt_pool_ctx_new_instance` tries to use one of these recycled
objects before allocating a new one from the heap.

libcork's memory pool also allocates data instances in batches, which we
don't currently do.  It might be worth doing some experiments to see if
this is a worthwhile addition.  (The context-aware recycling might be
enough of a benefit over malloc(3) on its own.)