No duplicate tyvars
Less quotes and shorter lines in error messages
Integrate with the Holes proposal

Wildcards in type signatures now cause errors as in standard Haskell, but
these error messages now inform the user of the types the wildcards were
instantiated to.

However, when the user enables the PartialTypeSignatures extension, the
inferred types of the wildcards will simply be used.
Remove unused function newWildcardTy + style