Types design#17

Why should the Trie itself be parameterized over key type? Currently both having `K` as type parameter and needing it passed as owned to `insert` and (especially) to `entry` pretty much forces us to make copy of key for each invocation. I think we only need key when first inserting into the trie, which could be better achieved with `ToOwned` (btw this comment makes sense for me: https://github.com/sdleffler/qp-trie-rs/blob/91507d5cfee53bafdeeb69deb36140f496a3bbaa/src/node.rs#L245). This way, `insert` and `entry` would require `Borrow` + `ToOwned` and would clone only when really necessary. Then if we stored `[u8]` in some form in node and parameterize functions like `insert` and `entry` themselves which would allow passing them `&'a [u8]` with different `'a` which is currently impossible.