refactor: pub visibility modifier

[TalDerei]

This is required, otherwise the monorepo will complain about some wrapper functions being private associated functions.

[cronokirby]

from_montgomery_limbs and from_raw_bytes shouldn't be exposed. The former is impossible to use correctly unless you really know the constants and what's going on internally. The latter should be replaced with the _mod_order version that's already exposed.

[TalDerei]

I kept to_le_limbs and from_le_limbs exposed. In the monorepo, we call Fq::from_le_limbs(hash.inner) where the input type is [u64::MAX; 4]. Isn't exposing from_le_limbs necessary in this case?

[cronokirby]

We absolutely do not want to allow doing that, instead, we should use the Fq::SENTINEL value

[redshiftzero]

We realized while modifying poseidon377 to work in embedded environments (ref: penumbra-zone/poseidon377#53) that we'll need Fq::from_montgomery_limbs to be pub in order to instantiate Fq constants. Upon discussion in Discord with @cronokirby it makes sense to add a disclaimer to the Fq::from_montgomery_limb methods that this should not be used unless you are familiar with the internals of the library (done in #98).

[TalDerei]

closing in favor of #98