Sparse ledger changes to support dynamic account creation

[mrmr1993]

This PR tweaks the sparse ledger implementation to

• track the number of allocated accounts in the ledger
• support allocating additional accounts
• support reading, writing, and getting paths for locations that are 'empty' (i.e. after the last allocated account).

To achieve this, Sparse_ledger.find_index_exn has been replaced with a new Sparse_ledger.find_index, along with a limited set of other changes.

Checklist:

• Dependency versions are unchanged
  • Notify Velocity team if dependencies must change in CI
• Modified the current draft of release notes with details on what is completed or incomplete within this project
• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? List them

[mrmr1993]

!ci-build-me

[mrmr1993]

!ci-nightly-me

[mrmr1993]

Breaking change; closing.