Message digests and hash maps/table implementations:

* md5: open source implementation based on Colin Plumb's code.

* sha2:  open source implementation by Aaron Gifford.

* ghash: Generic hash table

* xhash: Hash table with supports memcap and automatic memory recovery
  when out of memory.

* zhash: zero runtime allocations/preallocated hash table.

Use of the above hashing utilities is primarily for use by pre-existing code.
For new code, use standard template library and C++11 features.

For thread-safe shared caches:

* lru_cache_shared: A thread-safe LRU map.

