Fix bug #294: Crash when exprting record selectors of data family instan...

...ces

This also fixes a related but never-before-mentioned bug about the
display of GADT record selectors with non-polymorphic type signatures.

Note: Associated data type constructors fail to show up if nothing is
exported that they could be attached to. Exporting any of the data types
in the instance head, or the class + data family itself, causes them to
show up, but in the absence of either of these, exporting just the
associated data type with the constructor itself will result in it
being hidden.

The only scenario I can come up that would involve this kind of
situation involved OverlappingInstances, and even then it can be
mitigated by just exporting the class itself, so I'm not going to solve
it since the logic would most likely be very complicated.