Add keys_with_prefix and iterkeys_with_prefix methods

I noticed that luikore/hat-trie version adds some methods like hattrie_iter_with_prefix based on  dcjones/hat-trie version, so I replace the dcjones/hat-trie with luikore/hat-trie version, and add keys_with_prefix and iterkeys_with_prefix methods based on hattrie_iter_with_prefix. I dont know if want to have such a change? if not,  please ignore it.