In-memory prototype of MARF replace (SOAR-DB!)

[kantai]

Description

This PR implements an in-memory optimistic fork-aware data store, for replacing the MARF. The general idea with the datastore is to store the current data view as a normal key-value store and track the history of operations on the storage. When a fork occurs, the data state is unwound and then replayed.

This first PR just implements and tests a simple in-memory version of the data store, which unblocks work on integrating the db with the rest of the subnet node (that work is ongoing in the feat/soar-db branch.

[jbencin]

Overall very nice code! I've only got a couple minor comments:

• It seems like a lot of dependencies are declared in Cargo.toml but not used
• I'd recommend putting //! comments at the top of each file, it makes navigating documentation easier

[codecov-commenter]

Codecov Report

Merging #213 (4712f42) into master (54c653c) will increase coverage by 0.85%.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
+ Coverage   91.19%   92.04%   +0.85%     
==========================================
  Files           6        6              
  Lines         284      327      +43     
==========================================
+ Hits          259      301      +42     
- Misses         25       26       +1     

Impacted Files	Coverage Δ
core-contracts/contracts/helper/simple-ft.clar	76.47% <ø> (ø)
...contracts/contracts/helper/simple-nft-no-mint.clar	66.66% <ø> (ø)
core-contracts/contracts/helper/simple-nft.clar	73.33% <ø> (ø)
core-contracts/contracts/multi-miner.clar	97.05% <ø> (ø)
core-contracts/contracts/subnet.clar	94.75% <ø> (+0.61%)	⬆️

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[kantai]

Thanks for the review! I eliminated the unused dependencies (which aren't getting used in a subsequent PR) and also added rustdoc inline (//!) comments to the files.