Added a hashtable implementation

[latencyhiding]

I wrote a hashtable implementation, though I didn't fully integrate it with the rest of the code as that requires an API change. The main contribution was hash_table_t, a struct that you pass to hashtable* functions.

[latencyhiding]

Ok I made parse_obj grab the indices of the appropriate material from a hashtable after parsing with parse_mtl. The API is not touched.

I haven't tested this fully yet, but input would be appreciated!

[syoyo]

Thanks! Merged.