refactor: Witness API with MarshalBinary and MarshalJSON

[gbotrel]

Work in progress. Main changes:

Previously:
backend/witness package offered api like witness.WriteFullTo(writer) witness.ReadPublicFrom(reader)...

Now has a struct that implements encoding/BinaryMarshaler and encoding/json/Marshaler. Essentially the witness can be in 3 different "state":

1. frontend.Circuit --> this is referred to an "assignment"
2. []fr.Element --> this is represented under witness.Witness struct introduced by this PR
3. serialized (binary or json)

witness package enables to go from each of these states.

Other changes to validate approach:

backend/groth16 and backend/plonk had ReadAndProve and ReadAndVerify methods. Now Prove and Verify take a witness.Witness as parameter. Puts more work on caller (can't pass a frontend.Circuiit directly) but enables all workflow more cleanly.

This PR might be significantly simplified if we add generics.

Added round trip marshaling test (binary & json) in the test.Assert call.

Another PR should

• Change APIs of IsSolved (in backend/groth16 & backend/plonk) to take a witness.Witness and to be defined, if possible directly on the frontend.CompiledConstraintSystem interface, since it's not directly dependent on the backend.

[ivokub]

From the examples it makes sense to require that witness is instantiated separately from the circuit. I haven't tried it out myself, but from the examples the proposed interface looks good.

There are a few comments, but mostly looks good.