Support hashmap_get_by_hash, and hashmap_set_by_hash

[okanaganrusty]

As part of a project we're working on, we are not creating a key based on the item stored in the hash map. We have a pointer to the item which we need to lookup within the hash map at periodic intervals.

[scossu]

Maybe I'm missing something, but isn't this the same as defining a hash function such as

uint64_t my_hash_fn (
        const void *item, uint64_t seed0, uint64_t seed1)
{ return (uint64_t) item; }

for your hash map? (I am not grasping the use case, since if you are looking up something by pointer, you already know how to get it)

[ghost]

Considering there is the function hashmap_set_with_hash now, this PR could be closed...