Add Mnemonic type and key constructors from string

[notmandatory]

Description

Add the ability to create PublicDescriptorKey or SecretDescriptorKey from a string, including non-extended key types like WIF. An error is thrown if the string isn't a valid key.

Notes to the reviewers

The Mnemonic type also needs to be added back since only one constructor type can throw an error per the UDL limitations, and that means we need to type check the mnemonic words in the Mnemonic constructor.

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

[notmandatory]

Closing this PR in favor of #219 .