* Support for type families in LaTeX

The code is ported over from the XHTML backend.

* Refactor XHTML and LaTeX family handling

This is mostly a consolidation effort: stripping extra exports,
inlining some short definitions, and trying to make the backends
match.

The LaTeX backend now has preliminary support for data families,
although the only the data instance head is printed (not the actual constructors).

Both backends also now use "newtype" for newtype data family
instances.

* Add some tests