Move `Internal`s to separate package

[georgefst]

I'm not a big fan of the internal-module pattern.

This package (which AFAIK still has no users other than myself) might be a good place to experiment with the idea of using a separately-versioned inner package instead. This would give us better documentation on Hackage, and avoid the need to regard certain sorts of module names as special.

[georgefst]

There was an interesting discussion about this on Discourse. Unfortunately it revealed that the inability to separately version public sublibraries is a major shortcoming.