In order to be able to use a generic hashmap in things such
as the fs_backing_store we want one to exist. Here it is,
along with some moderately comprehensive tests.
Current limits:
1. All keys and values are owned by the hashmap
2. The hashmap, while capable of different bucket counts
only has a single fixed count for now
Signed-off-by: Daniel Silverstone <dsilvers@digital-scurf.org>
