Fix change/range proofs + simplify the code

[rrazvan1]

Why this should be merged

Fixing proof verification bugs:

1. exclusion proofs -> passing when providing a proof path which has the last node: a sibling of an ancestor of the existing node
2. exclusion proofs -> passing when providing not fully extended proof
   This issues were mainly affecting the simple proofs verification, but also change/range proofs.

And also, simplifying the code for better readability and maintainability.

How this works

• moving the main logic of proofs validation inside verifyProofPath(proofPath []ProofNode, key Key) and simplifying the specific proof verifications.
• added new specific errors
• more details within comments
• unifying change/range proof flows

How this was tested

UTs

Need to be documented in RELEASES.md?

[github-actions]

This PR has become stale because it has been open for 30 days with no activity. Adding the lifecycle/frozen label will cause this PR to ignore lifecycle events.

[joshua-kim]

nit: We can remove this line since the zero value is already nil... or not if you want to be explicit (I would personally leave it as nil but I can see the reasoning for not doing so). Leaving this to developer preference.

[StephenButtolph]

(didn't actually review, x/ shouldn't require multiple reviews.)