[api breaking changes]: added serialization for the witness

[gbotrel]

This PR started as an effort to enable inter-process / languages calls to gnark APIs.

Up until now, groth16 Witness was a map[string]interface{}. Using string for variable IDs was quickly inconvenient in non-Go codebases, and not needed.

For exposed APIs, here are the main changes:

• backend/groth16 now takes frontend.Witness as input, which is an alias for frontend.Circuit, instead of an untyped interface (previously backed by a frontend.Circuit or map[string]interface{}). It parses the circuit the same ways frontend.Compile does, guaranteeing the same order of the secret and public inputs.
• added backend/witness to serialize a frontend.Witness into []byte (through a io.Writer)

For internal packages:

• internal/backend/curveXXX/groth16 Prove and Verify takes []fr.Element as input .
• internal/backend/curveXXX/groth16.VerifyingKey now that named inputs are out, matches bellman.VerifyingKey enabling gnark to Verify bellman generated proofs for free (see groth16.Verify should be able to verify bellman-generated proof  #30 ). Note that pre-this PR serializations of VerifyingKey are not compatible with new serialization protocol / order.
• internal/backend/curveXXX/witness implements witness [de]serialization logic, ([]byte, frontend.Witness, []fr.Element)
• frontend/ all code related to input name management is gone.
• renamed "solution", "input", "assignment" to witness or publicWitness in all the code base