Changes:
- get rid of `extract_r1cs_and_z` and `extract_z`
- move `extract_r1cs` and `extract_w_x` from `frontend/arkworks` into
  `r1cs.rs`
The reasoning: they are not methods needed for the Frontend interface,
but only needed internally for the folding scheme to extract values from
the AugmentedF circuit and similar.
- set the `FCircuit` as the trait for the `src/frontend`
- remove the `frontend/arkworks` since the `FCircuit` trait can be
  directly implemented without a middle layer
- reorganize test circuits into `src/frontend/mod.rs`, updating them into
    `CubicFCircuit`: the typical x^3+x+5=y circuit
    `CustomFCircuit`: a circuit in which you can specify the number of
constraints that it will take
  where both fulfill the `FCircuit` trait, and they are used for
  different tests being folded.