This changes the actual structure of Mnemonic since it requires
exclusively owned types when implementing FromStr. Now, Mnemonic
contains an Arc. Thread safety is required because of the Tokio
multithreaded runtime, hence an Arc instead of an Rc.
This does add some level of burden for people instantiating Mnemonics,
but `Wordlist::arc(self) -> Arc<Self>` has been provided as a
convenience method to make working with mnemonics easier.