Add a check for circuit solvability at the end of compilation.

[TomAFrench]

We're currently getting a situation where some aztec contracts are having missing witness assignments: AztecProtocol/aztec-packages#9488

As a final compilation step we should add a check where we ensure that all opcodes are solvable. Starting from the known set of input witnesses we can ensure that all inputs are known for each opcode by the time we reach it, adding the outputs of that opcode to the new set of known witnesses.

This will ensure that there's continuity in the ACIR circuits we generate without needing to know the inputs.