New LeanIMT incremental Merkle tree

[cedoor]

Description

This PR moves the old incremental-merkle-tree library to the new imt package, renaming classes and files, and adds a new incremental Merkle tree library (i.e. LeanIMT), an optimized binary incremental Merkle tree.

The libraries' code has been documented and tests have been reorganized to meet high-quality requirements.

Related Issue

Closes #67

Does this introduce a breaking change?

• Yes
• No

TODO

• Add more comments to the old IMT implementation
• Rename incremental-merkle-tree.sol to imt.sol
• Refactor old Solidity implementations' tests
• Add LeanIMT Solidity implementation
• Add documentation to Solidity implementations' code
• Add warnings about new lib names

[0xjei]

Outstanding job! I truly commend your approach in modularizing and generalizing the code to the fullest extent. The organization is both meticulous and efficient, to be honest :) I've noticed a few minor oversights in the sample READMEs, but overall, fantastic work! 🤩