Migrate descriptor types to proc macros

[rustaceanrob]

Description

Move Descriptor, DescriptorPublicKey, DescriptorPrivateKey, DerivationPath, and Mnemonic to proc macros. Adds documentation when appropriate, including links. Uses uniffi::export(Display) to derive Display, think that is the proper way to do it?

Notes to the reviewers

There was a TODO questioning the use of thread_rng to create a mnemonic. Yes, thread_rng implements CryptoRng: https://docs.rs/rand/latest/rand/rngs/struct.ThreadRng.html#impl-CryptoRng-for-ThreadRng

Checklists

All Submissions:

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

[rustaceanrob]

751134c is rebased

[rustaceanrob]

2130c64 rebased once more

[thunderbiscuit]

I'm trying to decide if these are part of the API we agreed not to break as part of the 1.0, because the renaming of the keychain argument to keychainKind is a breaking change.

The removal of the as_string() methods in favour of the Display trait which would give us the better toString() would also be, but in this case we can simply have both and not create a break.

[thunderbiscuit]

Same as above: this is a vestige of the times where we could not get the Display trait on types.

I think we should add the Display in this case so the .toString() method is implemented for DescriptorPublicKey.

[thunderbiscuit]

This is a vestige of the times where we could not get the Display trait on types.

I think we should add the Display in this case so the .toString() method is implemented for DescriptorSecretKey.

[rustaceanrob]

I am not sure I'm following fully, meaning just derive Display? Or also remove this?