bench: Add reindex_tx_graph benchmark

[ValuedMammal]

This PR contains the following changes:

• Add benchmark reindex_tx_graph. Run with cargo bench -p bdk_chain --bench indexer.
• Add unit test to keychain_txout module to test behavior of spk-cache.
• Fixup a few doc comments.

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• I've signed all my commits
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• I ran cargo +nightly fmt and cargo clippy before committing

[ValuedMammal]

This is what I found compared to the same test (but no cache) on master:

	 Running benches/indexer.rs (target/release/deps/index-d7b15ff0fa7d9bb0)
reindex_tx_graph        time:   [649.59 us 656.47 us 667.48 us]                             
                        change: [-99.043% -99.038% -99.032%] (p = 0.00 < 0.05)
                        Performance has improved.

[oleonardolima]

tACK bb6d7c1

Thanks for the documentation fixup, got similar results when running locally:

Gnuplot not found, disabling plotting
reindex_tx_graph        time:   [320.04 us 320.93 us 322.03 us]
                        change: [-99.087% -99.084% -99.081%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 9 outliers among 100 measurements (9.00%)
  4 (4.00%) high mild
  5 (5.00%) high severe

Gnuplot not found, disabling plotting

[LagginTimes]

Looking good! I get the same benchmark results:

reindex_tx_graph        time:   [300.23 us 301.44 us 302.84 us]                             
                        change: [-99.286% -99.278% -99.268%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 16 outliers among 100 measurements (16.00%)
  7 (7.00%) high mild
  9 (9.00%) high severe

Just one minor suggestion:

[LagginTimes]

Suggested change

	assert_eq!(exp_spks.get(&i), Some(cached_spk));
	let exp_spk = desc.at_derivation_index(i).unwrap().script_pubkey();
	assert_eq!(&exp_spk, cached_spk);

Might be cleaner to remove the exp_spks on line 1138 entirely and do a direct lookup inside the loop.

[ValuedMammal]

Thanks @LagginTimes

[LagginTimes]

ACK cb55c51

[ValuedMammal]

Rebased this on #1978.

ACK f51f5b5