Potentially make `fields::nonnative::params` module private

[Pratyush]

• params has been used somewhere else: https://github.com/arkworks-rs/sponge/blob/c9f7d24a5f10c415e9370a079cb060d95400b938/src/constraints/mod.rs#L19 (bits_le_to_nonnative) and https://github.com/arkworks-rs/crypto-primitives/blob/1a71386a10ef627834e4c3b14ca982811f23192d/src/snark/constraints.rs (NonNativeFieldInputVar).

We should move NonNativeFieldInputVar and bits_le_to_nonnative into r1cs-std, and the external world should just call them.

Originally posted by @weikengchen in #79 (comment)