Derive Generic instance for Either

[paulyoung]

purescript-deprecated/purescript-generics-rep#34 (comment) says:

The instance for Either should live in purescript-either instead, and should be derivable.

I'm not sure if the concerns expressed in that other PR apply here.

[hdgarrood]

This one does still apply:

I’d prefer to consider all of the data types across the core libraries and come up with a uniform approach for making sure all of the types which should have a Generic instance do.

[paulyoung]

@garyb had a suggestion along those lines here: purescript-deprecated/purescript-generics-rep#43 (comment)

[JordanMartinez]

We'll be moving the generics-rep repo's code into prelude, so we can drop the dependency on generics-rep.

[JordanMartinez]

Superceded by #61, which builds off this PR but removes the generics-rep dependency because it was ported to prelude.