Plonk: helper function plonk_prepare_matrix to build gates matrix

[vesselinux]

Implement helper function plonk_prepare_matrix to:

1. Prepare a gates matrix that has the top $L$ rows corresponding to $L$ Plonk public input gates (for a circuit with $L$ public inputs)

The caller will then manually:

1. Fill in the remaining rows from $L+1$ to $N$ (for a circuit with $N$ gates) of the gates matrix with the remaining Plonk gates describing the circuit (add, multiply, multiply-by-constant, dummy)

2. Generate the wire permutation (copy constraints) including the public inputs

3. Pass the constructed gates matrix, together with the wire permutation and public input indices as an input to function plonk_srs_derive_from_usrs (see Issue Plonk: update inputs to plonk_srs_derive_from_usrs(usrs, matrix, perm, pi_indices) #88)

[dtebbs]

First step towards #95