Implements a sparse k-ary Merkle tree

[howardwu]

Thank you for submitting the PR! We appreciate you spending the time to work on these changes!

Motivation

Implements a sparse k-ary Merkle tree in console/collections and circuit/collections.

• Supports BHP and Poseidon in full
• Restricts Keccak/SHA3 from being used as a key
• Supports Keccak/SHA3 as a leaf hasher and path hasher

Test Plan

Includes tests on all methods for the supported hash function variants.