WyC: Very minor tweak to prevent char negation #418.

It never makes sense to have a negative character value.  Therefore, I
have prevented the possibility by disallowing char type for use with the
negation operator.
WyC: Updated more test cases for #418.

Have made additional changes to test cases which are invalid under
changes for #418.  Typical modifications include inserting casts to make
previously implicit coercions explicit.  There are still a number of
outstanding problems, however.