Summary: Add a couple benchmarks measuring allocation/deallocation latency.  One benchmark measures an allocate-deallocate repeated pattern, the other measures the allocation of a large chunk of fibers.

We'll use these benchmarks later on after we modify the allocation/deallocation behavior of fibers.

Reviewed By: pavlo-fb

Differential Revision: D2573064

fb-gh-sync-id: 414eb93d6223e42c187c03214a47dfb533491bab