[chain] Create module `indexer`

[ValuedMammal]

Introduce module keychain_txout_index and replace old keychain module with balance.rs

fixes #1317 replaces #1175

Changelog notice

Removed keychain module from bdk_chain. The Balance type is now re-exported from the top level

Checklists

All Submissions:

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

[LLFourn]

I wonder what we think about:

indexer.rs // the trait in here
indexer/
  ├── keychain_txout.rs
  └── spk_txout.rs

[ValuedMammal]

@LLFourn Should indexer be a pub mod or do we just re export everything under it?

[storopoli]

I think re-export, despite pedantic, is more future-proof.

[ValuedMammal]

Rebased onto a112b4d9

[LLFourn]

@LLFourn Should indexer be a pub mod or do we just re export everything under it?

I think it must be pub mod otherwise you will be re-exporting the ChangeSet in the keychain_txout.rs file at the root! I would just make people import by indexer::keychain_txout::KeychainTxOutIndex.

I wouldn't mind dropping the TxOut from these which seems pretty redundant.

[ValuedMammal]

• Fixed file indexer.rs
• Made indexer a public module

[notmandatory]

I'd prefer to merge this as is rather than doing any more renaming at this point.