Better error messages for invalid class declarations
Better error messages for invalid class declarations

Check whether the typeclass has one valid parameter. 
Reject classes with zero or multiple parameters with
a clear error message.

Renamer is not the best place to do this, but it must
be done before kind inference, so there are not very
many alternatives.

shinnya/jhc-mirror,hvr/jhc,shinnya/jhc-mirror,shinnya/jhc-mirror,hvr/jhc,shinnya/jhc-mirror,hvr/jhc,hvr/jhc