feat: enforce unique account prefix

[tomyrd]

closes #601

[bobbinth]

Not a review yet - but one thing I'm wondering is whether we should tackle this issue in a broader context. Specifically, currently account tree uses a SimpleSmt and so the methodology for checking uniqueness relies on this. But it is probably a good idea to migrate from SimpleSmt to a regular Smt - and in this case, we probably would need to modify the approach somewhat.

We should still weigh all pros and cons of migrating from SimpleSmt to Smt - but it may make sense to put this PR on hold until we make a decision here.