Reworked hashmap growing/rehashing behavior

- Now the hashmap grows only when actual live entries (i.e. not
  tombstones) occupy at least 37.5% (half of occupancy threshold - 75%) of the buckets
  after a successful `EXT_HMAP_NEEDS_RESIZE` check (that checks if live entries +
  tombstones exceed the 75% threshold). This should make the growing behaviour a little
  bit more conservative in mixed usage scenarios (lots of inserts, deleted and lookups).
- Swapped old c-string hash function with FNV-1a Hash. This gives us a slightly better
  distribution at no extra downsides.

Author: bamless