Introduce `lookahead` for `KeychainTxOutIndex`

[evanlinjin]

closes #163

The lookahead is the number of scripts to cache ahead of the last derived script index.

Scanning methods now return DerivationAdditions and replenish the internal lookahead.

TODO

• [x] Add lookaheads(&self) -> &BTreeMap<K, u32>
• [x] Rebrand store.. methods as set_derivation_index
• [x] Better documentation
• [x] Tests

[codecov-commenter]

Codecov Report

Merging #169 (f9776b1) into master (5cb0fd5) will increase coverage by 1.59%. The diff coverage is n/a.

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@@            Coverage Diff             @@
##           master     #169      +/-   ##
==========================================
+ Coverage   54.83%   56.43%   +1.59%     
==========================================
  Files          10        9       -1     
  Lines         248      241       -7     
==========================================
  Hits          136      136              
+ Misses        112      105       -7     

Impacted Files	Coverage Δ
bdk_chain/src/chain_graph.rs	81.81% <ø> (ø)
bdk_chain/src/keychain.rs	56.89% <ø> (ø)
bdk_chain/src/tx_data_traits.rs	100.00% <ø> (ø)
bdk_chain/src/tx_graph.rs	75.00% <ø> (ø)
bdk_chain/src/keychain/keychain_txout_index.rs

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[evanlinjin]

@LLFourn

• I had another go at renaming (I think it is a lot better now).
• reveal.. methods now returned iterators of things they have newly revealed.
• I repurposed descriptor_into_script_iter into range_descriptor_spks where you can specify a range of indices (and the descriptor is wrapped in Cow to either be owned (for external use) or borrowed (for internal use)). This way it can be used efficiently within reveal.. methods and replenish_lookahead while enforcing checks (such as non-wildcard descriptors should only have spk at 0) for them. We want to use this as a wrapper because miniscript library allows deriving non-wildcard descriptors above 0 (which returns the same script for every index) and this resulted in a bug).
• Updated tests and docs further.