Simplify creating types from bytes (remove default()/copy_from_slice()/from_repr())

[conradoplg]

To create certain types from bytes we need to go through a bunch of steps, e.g.

let mut repr = <S::Scalar as PrimeField>::Repr::default();
repr.as_mut().copy_from_slice(&s_bytes);
let maybe_scalar = S::Scalar::from_repr(repr);

It seems this could be improved by const_evaluatable_checked which has not stabilized yet. Check if that is the best solution or if there is an alternative, and implement it if possible.