TL;DR: splits off one major source of diff lines from PR microsoft#283. Inherently, there are many R1CS shapes to consider when tailoring public parameter creation to non-uniform step-circuits.
However, the commitment key generation should only be done once for all circuits (once a suitable size has been determined by looking at all R1CS shapes). This splits the relevant Nova functions
into `r1cs_shape_and_key`, `r1cs_shape` and `commitment_key` to enable the flexibility deamnded by the above model.

In detail:
- Renamed the `r1cs_shape` method across various files to `r1cs_shape_and_key`, indicating its functionality is to return both `R1CSShape` and `CommitmentKey`.
- Altered function calls from `r1cs_shape` to `r1cs_shape_and_key` in files such as `direct.rs`, `nifs.rs`, `lib.rs` and `circuit.rs`,
- Split the creation of `R1CSShape` and `CommitmentKey` into separate functions in the `NovaShape` object in `r1cs.rs`
- Removed the `R1CS` struct in `mod.rs` as it only contained a phantom data, with related operations performed elsewhere.
- Implemented changes to enhance code readability, including the addition of a new `commitment_key_size` function, and overall code reformatting for clarity.