Remove code duplication for `to_bytes` and `from_bytes`

[moCello]

Summary

With the implementation of the trait Serializable the to_bytes and from_bytes methods are duplicated for some types.
If the function body is (mostly) identical, it is better to not duplicate the code but to use the upstream implementations of the methods.

This seems to be the case for the types Scalar, G1Affine and G2Affine.