Fix GNFData instance for V1

[treeowl]

Make grnf for V1 a well-defined function that forces its
(bottom) argument rather than making the function itself bottom.

Fixes #19

[RyanGlScott]

This looks reasonable to me. It might be worthwhile to add a note to the CHANGELOG.

Any objections to merging this, @hvr?

[hvr]

@RyanGlScott no objections :-)

I'm wondering as well if we want to mention this in the changelog... (since we're both considering it, we just should)

[RyanGlScott]

@hvr I think it's worth mentioning, since it changes the way generically defined NFData instances behave. Before, they would unconditionally throw an error, regardless of whether the argument to grnf is let x = x in x or undefined. Now, if the argument truly does recurse infinitely when evaluated, then calling grnf will actually recurse infinitely, so you won't be able to observe this behavior with something like an exception handler.