Add context to range proof

[zaverucha]

Add support for a context value for the range proof. Then when calling it during show, pass the challenge from the DLEQ proof we create in show_groth16 (the challenge is in the struct ShowProof.show_groth16.pok_inputs.c). This may not be strictly necessary, but it seems prudent to bind the range proof to the rest of the show proof.

crescent-credentials/creds/src/rangeproof.rs

Line 419 in 8e901b0

None, // TODO: should we add a presentation message here?

crescent-credentials/creds/src/rangeproof.rs

Line 233 in 8e901b0

None, // TODO: should we add a presentation message here? (use the c from the dlog proof?)