POC: Multi-descriptor wallet

[evanlinjin]

Closes #1536

Description

Based on the discussion about single-keychain wallets on Tuesday, I couldn't stop myself from experimenting with an any-keychain-count wallet. This is done by introducing a type parameter to Wallet<K>.

This is obviously a breaking change, but this seems to be the only non-hacky solution for the problem. The only downside is that a whole bunch of wallet methods now return Option since we can have keychains without corresponding descriptors.

TxBuilder parameters have been refactored to make sense in this context.

ChangeSet has been refactored to allow for any keychain count.

Notes to the reviewers

It's building with cargo build -p bdk_wallet. The tests do not compile.

To see something that has a bigger chance of being merged, check out #1533.

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]

Concept ACK for the 2.0 milestone.

For 1.0 the priority should be on API stability which #1533 accomplishes in a less disruptive way.

[evanlinjin]

Closing because I can't see this coming to light.