-------------------------------
      A day's work to improve error messages
	-------------------------------

1.  Indicate when the cause of the error is likely to be the monomorpism
    restriction, and identify the offending variables.  This involves
    mainly tcSimplifyTop and its error generation.

2.  Produce much better kind error messages.  No more
      ../alonzo/DiGraph.hs:40:
	  Couldn't match `* -> *' against `Type bx'
	      Expected kind: * -> *
	      Inferred kind: Type bx
	  When checking that `DiGraph n' is a type

It took a surprisingly long time to get the details right.