feat: expose Wallet::unmark_used

[reez]

Description

This lets users undo mark_used calls. We already expose mark_used, so we are just adding unmark_used here.

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

[ItoroD]

ACK d9c5bc7

[thunderbiscuit]

ACK d9c5bc7.