Incremental Merkle Tree Implementation

[stechu]

Description

This PR implements an incremental merkle tree data structure. This is an append only merkle tree that only maintains Log N internal nodes. An invariant this implementation tries to maintain is that all operations is Log N.

closes: #69

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Before we can merge this PR, please make sure that all the following items have been checked off. If any of the checklist items are not applicable, please leave them but write a little note why.

• [x] Targeted PR against correct branch (main)
• [x] Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• [x] Wrote unit tests
• [x] Updated relevant documentation in the code
• [x] Added a relevant changelog entry to the Pending section in CHANGELOG.md
• [x] Re-reviewed Files changed in the Github PR explorer

[tsunrise]

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[stechu]

Since @daira's concern about the inner node empty value is not specific to this PR. Please let me know whether I should address her concern in this PR or not. @tsunrise @weikengchen @Pratyush