Split instances out for Generic into a standalone library

[garyb]

Since the situation with generics is still up in the air. We can create a generics-rep version too. It'll need newtypes or something, to prevent the instances being orphaned, however (or use of the gEncodeJson / gDecodeJson directly).

[dgendill]

For anyone looking for the old generic gEncodeJson and gDecodeJson functions, it looks like they are in purescript-argonaut-generic now.

[garyb]

It now has a versioned release too, so closing this :)