Add Default instance for Bool

[ndmitchell]

Currently it's missing. Should return False.

[cdsmith]

I just ran into this. I appreciate that there's a lot of sentiment against using data-default in the first place, or against most of its instances. Nevertheless, I see no rational argument at all for having instances for Int, String and even a -> b, but not Bool. This seems like a clear oversight that should be corrected.