Update removed nodes

[vplasencia]

Description

This PR:

• Adds a check to the _update function in the InternalLeanIMT.sol file to update the newLeaf index only if newLeaf is different from 0. Now when trying to update a removed leaf the transaction will revert with the custom error LeafDoesNotExist.

• Removes unnecessary newLeaf != 0 check. Since the _has function will no longer return true for the leaf 0, this check is no longer necessary.

• Adds new tests to cover 100% of the lines of InternalLeanIMT.sol,

Related Issue(s)

Closes #228

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have run yarn prettier and yarn lint without getting any errors
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules

[0xbok]

looks good. i'd also update/add a test where after deleting a leaf, self._has(0) is asserted to be false.

[vplasencia]

Hey @0xbok! I think that's covered in the test Should not update a leaf that was removed, but it makes sense to add one just for it in the has section. Thanks.

[vplasencia]

@0xbok done ✅