Timber updates sibling paths using incoming leaves.

[IlyasRidhuan]

This closes #581 and is proof that #540 can be solved statelessly. Activating this functionality across client, optimist and wallet is sufficiently large and complex to warrant another PR.

This PR allows Timber to statelessly update a sibling path for a leaf index to account for the insertion of new leaves. This removes the restriction that spending commitments must reference the block these commitments were "mined" in.

Unit/Property tests (Check Sibling Path Increment) are included and can be run using
npx mocha --timeout 0 --bail --exit test/timber.test.mjs

Note There is duplication as the common-files update must also happen in the wallet directory. These can't share the same timber file due to a restriction in create-react-app including files outside of wallet/src