add hiding property for PCS

interesting, if we want a general API/traits, why not consider the cases where we do need hiding?

If we consider hiding, then two things:
- our `Polynomial` better be a [`LabeledPolynomial`](https://docs.rs/ark-poly-commit/0.3.0/ark_poly_commit/data_structures/struct.LabeledPolynomial.html) (at least something of similar spirit?)
- we need to pass in an `rng` to `commit()` etc.

_Originally posted by @alxiong in https://github.com/EspressoSystems/hyperplonk/pull/43#discussion_r927638129_