Feature request: Add compile API from to build circuit from source and assignments

[paulcadman]

vamp-ir is used as a library in the taiga product. It will be used to compile and execute circuits provided from user input in transactions. Both the vamp-ir code for a circuit and its inputs will be fields on some transaction data structure rather than files in the file-system.

It would be convenient to have a vamp-ir compile API to support this use case.

As a starting off point for discussion let's consider:

compile_from_source(
        vamp_ir_source: &str,
        named_field_assignments: HashMap<String, Fp>
        field_ops: &dyn FieldOps,
        config: &Config) -> Result<Halo2Module<FP>, VampIRError>

This could be made generic on the PrimeField (and module type? - not sure I don't know rust well).

requirements / questions / comments:

• The new API should not panic for errors we'd like to log/report, e.g parsing errors or invalid assignment errors should be captured in the VampIRError type.
  • For the last point I've proposed something for taiga for now. https://github.com/anoma/taiga/blob/aa80600f8e4f6c19bc3e75b812f3b9f615e51bcf/taiga_halo2/src/circuit/vamp_ir_utils.rs#L35 , an error is reported if there is a missing assignment but no error/panic occurs if there are variables with no name.
• Is there a better type than &str for the input source? Like a reader type that can support files and bytes?
• See from_vamp_ir_source I'm proposing in taiga https://github.com/anoma/taiga/blob/aa80600f8e4f6c19bc3e75b812f3b9f615e51bcf/taiga_halo2/src/circuit/vp_circuit.rs#L513