Add TxBuilder step 1: CoinControl

[danielabrozzoni]

Description

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

[danielabrozzoni]

Replying to @evanlinjin's comment https://gist.github.com/evanlinjin/41e6a23cc08e888884bded327e290866?permalink_comment_id=4587231#gistcomment-4587231 here:

• Let's make a PR soon! It's easier to discuss that way (like on the PR).
• Let's disallow duplicate UTXOs. Need some sort of filter HashSet or BTreeSet.
• It will be better to use utxos: impl IntoIterator so the caller won't need to .collect first if their UTXOs is not a Vec.

ACK

• add_planned_utxos() essentially forces the caller to group UTXOs using the same descriptor + assets together first. Would this be a problem?

I don't think it is, especially since I have a new list_unspent_by_keychain method in the wallet. But if you see any better method to do this, lmk!

Thoughts on spec-writing in general

Agreed, I'll try to apply those heuristics to the txbuilder docs, thanks!