Create custom Emulator backend

[uhbif19]

As we see CLB is too slow for fuzzing #60 and it does not seem to be fixable.

Due to most of Tx correctness checks should be validated by our off-chain code anyway (blocked by #65), emulator can be implemented by only recording current UTxO state. That is: if we do not count Plutus fees, which would be out of scope of this task.

TODO:

• Implement UTxO store and its querying
• Add support for ADA selection and minada enforcement without makeTransactionBodyAutoBalance
• Support submitting
• Support time
• Parameterize tests around backends

[github-actions]

Status: Blocked ❌

Issues blocking this PR:

• Cover all Tx submission errors in monads for greater DX #65 ❌

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