Expose leaf_sum and allow binary MerkleTree to be built from existing precomputed leafs

[AurelienFT]

This change allows us to run leaf_sum for each leaf before creating the tree and call new_from_existing_leafs using the precomputed leafs.

It give the possibility to the caller to optimize the computation of all leaf_sum beforehand by parallelising for example.

Checklist

• Breaking changes are clearly marked as such in the PR description and changelog
• New behavior is reflected in tests
• If performance characteristic of an instruction change, update gas costs as well or make a follow-up PR for that
• The specification matches the implemented behavior (link update PR if changes are needed)

Before requesting review

• I have reviewed the code myself
• I have created follow-up issues caused by this PR and linked them here

[rymnc]

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