Improve API for circuit (de-)compression

[moCello]

Summary

When decompressing a circuit, we immediately compile the circuit and return the prover and verifier, and because of this we require the label and publicparameter at decompression time which is counter intuitive. At the same time we compress the circuit as part of the compile module which is also counter intuitive. We can improve the API as follows: Current API:

let compressed = Compiler::compress::<DummyCircuit>().unwrap();

let (decompressed_prover, decompressed_verifier) =
    Compiler::decompress(&pp, label, &compressed).unwrap();

Proposed new API:

let compressed = DummyCircuit::compress().unwrap();

let (decompressed_prover, decompressed_verifier) =
    Compiler::compile_with_compressed(&pp, label, &compressed).unwrap();