Implement `KeychainTxOutIndex::apply_additions`

[evanlinjin]

Internally, this just uses store_all_up_to, however this will change once we start storing a "lookahead" index for each keychain.

Additionally, I have removed AsRef and AsMut implementations for DerivationAdditions, and DerivationAdditions now has a single field last_derived.

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This is my attempt at addressing https://github.com/LLFourn/bdk_core_staging/pull/154#issuecomment-1411479890

[codecov-commenter]

Codecov Report

Merging #160 (f846d7b) into mark-and-unmark-used (8438e30) will decrease coverage by 0.26%. The diff coverage is 100.00%.

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@@                   Coverage Diff                    @@
##           mark-and-unmark-used     #160      +/-   ##
========================================================
- Coverage                 65.89%   65.64%   -0.26%     
========================================================
  Files                         9        9              
  Lines                       390      390              
========================================================
- Hits                        257      256       -1     
- Misses                      133      134       +1     

Impacted Files	Coverage Δ
bdk_chain/src/keychain/keychain_txout_index.rs	0.00% <ø> (ø)
bdk_chain/src/keychain.rs	61.19% <100.00%> (-1.50%)	:arrow_down:

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[LLFourn]

Internally, this just uses store_all_up_to, however this will change once we start storing a "lookahead" index for each keychain.

I imagine that we will delete store_all_up_to and apply_additions will just take all its logic.

[LLFourn]

Can you make this PR to #154 instead?

[LLFourn]

I merged this into #154 already