Add API to build a circuit from vamp-ir source

[paulcadman]

There is only vamp-ir API for creating circuits inputs from a JSON file. Taiga will
need to create inputs from note data stored in partial transactions. So
we'll need API in vamp-ir to do this.

This PR adds API on VampIRValidityPredicateCircuit, for compiling a vamp-ir file from source.

from_vamp_ir_source(
        vamp_ir_source: &str,
        named_field_assignments: HashMap<String, Fp>,
    ) -> Result<Self, VampIRCircuitError>

In vamp-ir_utils I've also added API missing from the vamp-ir library which we can add to vamp-ir later.

get_circuit_assignments(
    module: &Module,
    named_assignments: &HashMap<String, Fp>,
) -> Result<HashMap<VariableId, Fp>, VariableAssignmentError>

and

parse(unparsed_file: &str) -> Result<Module, String>

[XuyangSong]

lgtm