TODOs in `plonk/proof_system/prover.rs`

[Autoparallel]

Prover

We need to implement:

• [ ] copy_placeholder()
• [ ] add_polynomial_evaluations_to_transcript() (not defined in barettenberg)
• [ ] compute_batch_opening_polynomial() (not defined in barettenberg)
• [ ] init_quotient_polynomial()
• [ ] compute_opening_elements()
• [ ] get_circuit_size()

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May need more help and comments here for the functions not implemented in barettenberg.