Refactor MerkleTreeGadget implementation

[mike1729]

Description

There are two implementations of MerkleTreeGadget for RescueMerkleTree and RescueSparseMerkleTree that are equivalent except types they use. Recently a bug was found in the impl for RescueMerkleTree that affected also the other and only one was fixed.
This PR proposes a refactor to a common implementation for both types.

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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.

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

[CLAassistant]

All committers have signed the CLA.

[mike1729]

There is still some work to be done in moving some tests around but I wanted first to check if you like the direction of the change.

[mrain]

There is still some work to be done in moving some tests around but I wanted first to check if you like the direction of the change.

Thanks very much for spotting this. It will be very helpful if you proceed ahead.
Much appreciated!

[mike1729]

Ready for review :)

[mrain]

Shoule delete this file?

[mrain]

LGTM in general, minor comments above.

Also do you think it's better to rename sparse_merkle_tree.rs as universal_merkle_tree.rs?

[mike1729]

@mrain I've addressed all comments :)

[mrain]

LGTM. Thanks a lot!