audit the useage of `RngCore + CryptoRng` vs `Rng` vs `CSPRNG`

[zhenfeizhang]

this triggers me to think if we should replace some RngCore + CryptoRng trait bound in our API to just Rng?

1. We are not implementing an PRNG ourselves, so we should probably always use Rng over RngCore, (since the latter is for backend implemented by the generator)
2. What are some places where we have to use CSPRNG? probably not in schonrr or transcript?

Originally posted by @alxiong in https://github.com/EspressoSystems/jellyfish/pull/53#discussion_r851586106

[alxiong]

Some follow-up conversations here: https://github.com/RustCrypto/traits/issues/1087