Change root.clone() -> mem::swap

[yejiayu]

I plan to use rc and refcell rewrite to avoid all clone.

[ava57r]

mem::swap fine works =). No?

[yejiayu]

https://github.com/cryptape/cita-trie/pull/18#discussion_r264109426

[ava57r]

String support error https://doc.rust-lang.org/std/string/struct.String.html function from_utf8 return error with bytes

pub struct FromUtf8Error {
    bytes: Vec<u8>,
    error: Utf8Error,
}

create type error with return Node.

pub struct TrieError {
    node: Node,
    error: Error,
}

[yejiayu]

@andreevlex I rewrote the entire library. #43