message: Fix type of public_input_size
diff: @@ -251,7 +251,7 @@ struct SnarkyConstraintSystem<Field, RustGates> {
  /** The row to use the next time we add a constraint. */
  next_row: usize,
  /** The size of the public input (which fills the first rows of our constraint system. */
  - public_input_size: usize,
  + public_input_size: Option<usize>,
  /** Whatever is not public input. */
  auxiliary_input_size: usize,
  /** Queue (of size 1) of generic gate. */
  