Add longest_prefix method

[Tessil]

Add a longest_prefix method to find the key in the trie which is the longest prefix of a given value (#13).

tsl::htrie_set<char> map = {"/foo", "/foo/bar"};

map.longest_prefix("/foo"); // returns "/foo"
map.longest_prefix("/foo/baz"); // returns "/foo"
map.longest_prefix("/foo/bar/baz"); // returns "/foo/bar"
map.longest_prefix("/foo/bar/"); // returns "/foo/bar"
map.longest_prefix("/bar"); // returns end()
map.longest_prefix(""); // returns end()