Replace ToString with Display for Mnemonic

[eupn]

Implement Display instead of ToString for Mnemonic.

From ToString docs:

This trait is automatically implemented for any type which implements the Display trait. As such, ToString shouldn't be implemented directly: Display should be implemented instead, and you get the ToString implementation for free

Aside from a code being more idiomatic, the user will get the ability to use mnemonics in format! parameters:

let mnemonic = <...>;
format!("Write down your mnemonic words: {}", mnemonic);

[stevenroose]

Could you try implement this without allocations? That should be quite straightforward. Just iterate over the words and write!( them one by one with a space in between.

[eupn]

Sure!