Check for account ID prefix uniqueness during block proving

[PhilippGackstatter]

What should be done?

After #1157, we can check for account ID uniqueness.

How should it be done?

As part of building ProposedBlock, in AccountUpdateAggregator::aggregate_account, we can check for uniqueness by inspecting the SmtLeaf of the SmtProof. If it is not of variant SmtLeaf::Empty, this would be an error. (This assumes the witness would contain such a proof after the account tree is built on top of Smt).
Later we'll have to also implement this check in MASM, but that's a separate issue.

When is this task done?

Once implemented and existing tests pass and a new test checking for the error condition has been added.

Additional context

No response

[bobbinth]

The idea here is to make sure no two account share the same 64-bit prefix, right?

[PhilippGackstatter]

The idea here is to make sure no two account share the same 64-bit prefix, right?

Yes, exactly. The title was a bit imprecise so I changed it.