internal_repr API convention

[vlopes11]

Currently we rely on internal_repr of some structures to get the internal representation of the type. Example:

bls12_381/src/fp.rs

Line 316 in 1503274

pub const fn internal_repr(&self) -> &[u64; 6] {

This is undesirable because it is not according to the Rust API standards. We should discuss a convention and replicate it to our stack.