Add descriptor checksum to wallet

[rustaceanrob]

Description

Adding from methods suggested in #596

Notes to the reviewers

public_descriptor was giving me grief when I tried to test it in python, so I'm starting simple here

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

[reez]

This will be a nice thanks for adding 👍

[thunderbiscuit]

ACK 53d654c.

Thanks!