Feature Request: Stepwise SubTrie API#40

I'd love it if the SubTrie API were more efficient, and if it were a slightly lower level abstraction of what actually happens. If I construct a Trie like ```rust let mut trie = Trie::new(); let map_insert = |map: &mut Trie<Vec<u8>, u32>, key: &str, value: u32| { map.insert(key.as_bytes().to_vec(), value); }; map_insert(&mut trie, "abc", 1); map_insert(&mut trie, "abcd", 2); map_insert(&mut trie, "abcde", 3); ``` Then I'd expect the following to be true ```rust trie.subtrie(&"abc".as_bytes()[..]) == trie.subtrie(&"a".as_bytes()[..]).subtrie(&"b".as_bytes()[..]).subtrie(&"c".as_bytes()[..]) ``` But the current subtrie API instead wants the entire prefix, over and over again. I'd also enjoy it if there were a function akin to ```rust trie.subtrie(&"abc".as_bytes()[..]).is_terminal() // or trie.subtrie(&"abc".as_bytes()[..]).get_value() // returns an Option<V> with the value if this is a "terminal" node ``` My use case is parsing a stream, character by character. There, it's nice if I do not have to keep track of already parsed characters to satisfy the Trie API. Part of why I really want this is the fact that libraries like [chumsky](https://github.com/zesterer/chumsky) and [combine](https://github.com/Marwes/combine) very much prefer it when one does the parsing step with individual characters, and doesn't look back at what has already been parsed.