Add AccountTree and PartialAccountTree

[PhilippGackstatter]

What should be done?

Add types for AccountTree and PartialAccountTree to miden-base.

How should it be done?

We have to migrate from SimpleSmt to Smt as the underlying data structure used for the account tree, and we should probably add strongly-typed wrappers for these instead of using bare Smts (like we do in MockChain, for example).

When is this task done?

When the above structures are implemented and the places where we use them are updated, which includes at least:

• MockChain
• LocalBlockProver

Simple unit tests can be considered for them, but they are mostly tested as part of LocalBlockProver, so this seems somewhat optional.

Additional context

No response