Add 'lsm' shell to work with lsm trees on the command-line

[lambdanature]

Implement command-line shell for lsm-tree to create and interact with lsm files. Update README with usage instructions and add tests for tool commands.

Note: The lsm tool is gated via the 'tool' feature to not pollute the dependencies of the crate's [lib] target.

The tool can be built, tested and installed with the following commands:

cargo build -F tool
cargo build --release -F tool
cargo test -F tool --test tool_tests
cargo install --path . -F tool

[codecov]

Codecov Report

❌ Patch coverage is 90.52419% with 47 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/tool.rs	90.52%	47 Missing ⚠️

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