Avoid storing hashing function pointers in storage make MerkleTree structure upgrade-safe

[Amxx]

Fixes #5079

This is change to an unreleased feature, no need for a changeset entry.

PR Checklist

• Tests

[changeset-bot]

⚠️ No Changeset found

Latest commit: 7d19d75

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[ernestognw]

I expanded a bit on the documentation, I felt we needed to give an official recommendation of how to store the function as it may seem pretty easy to just put it in storage as we were doing.

Also, I added cross-references where possible but the custom setup and push can't be referenced since adding two - (hyphens) gets encoded as —%E2%80%8B and can't find where the issue is (likely in https://github.com/OpenZeppelin/solidity-docgen/)

LGTM