`PrimeField::Repr` ambiguous endianness

[jonathanpwang]

It is unsafe in downstream use cases for PrimeField::to_repr and PrimeField::from_repr to have ambiguous endianness. (E.g., it's oftentimes hard to implement something for ff::PrimeField without assuming something about endianness.)

Can you either add an ENDIANNESS const or functions like to_bytes_le and from_bytes_le?