Reworked hashmap growing/rehashing behavior

- Now the hashmap grows only when actual live entries (i.e. not
  tombstones) occupy at least 50% 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, and it shows a ~50% increase in performance in
  mixed usage scenarios (lots of inserts, deleted and lookups).
- Also, the re-hash only path is now done inline instead of allocating
  new arrays of the same size. This mean that we have to do 2 passes
  over the hashes array instead of 1, but saves memory and reduces
  eventual heap fragmentation.
- Swapped old c-string hash function with FNV-1a Hash. This gives us a
  slightly better distribution at no extra downsides.

Author: bamless