Plonk: code cleanup

[vesselinux]

• Have explicit struct members of the a,b,c polynomials currently stored as a single vector W_polys_blinded_at_secret_g1[a,b,c] in the plonk_proof class. Similarly, have explicit struct members for the q_L, q_R, q_O, q_M, a_C selector polynomials currently stored as a single vector Q_polys in the circuit struct.

• In all header files: leave only functions that are externally visible to calling code. If a function is only used "internally" in the implementation then just leave it in the .tcc (and not in the .hpp), potentially in the internal namespace (to hide it from other code). See also discussion: [BASE] Plonk #49 (comment)

• Make sure that all functions do not rely on the caller for allocating the correct sizes of the return parameters. Make the latter to be allocated inside the functions that compute them. See also [BASE] Plonk #49 (comment)

[vesselinux]

Just to confirm regarding bullet 1 above: in a previous discussion we weren't of a strong opinion whether this should be implemented now or later or if at all. Should I proceed with implementing it now, or shall we leave it as an optional TODO for later?

[vesselinux]

Added two fixes regarding bullet 7. Did not find other cases.

To the best of my abilities checked that the code complies to the rule in bullet 4. Did not find any violations, but may be worth discussing.