Hashmap

[adamhutchings]

We need some hash map utilities. Probably have the keys be const char *s and the values be void *. Here is a suggested API:
Creating and destroying:
int hm_init(Hashmap *hm, int num_buckets);
int hm_free(Hashmap *hm);
Looking up and setting values:
void* hm_lookup(Hashmap *hm, const char* key);
int hm_set(Hashmap *hm, const char *key, void *value);
Alter this as you see fit, as long as we can use const char* keys to access void* values and it mostly runs in O(1).