Add display trait to mnemonic

[reez]

Description

Adds more to #551 , I was working on BDK iOS and just saw that I was using Mnemonic.asString() so I wanted to make that nicer by using the display trait on Mnemonic instead now after seeing #551 .

This also removes the asString() method which is nice.

Notes to the reviewers

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[thunderbiscuit]

ACK 94d31ff. Awesome to see how easy it is to convert our current structs to the new way of doing it.