Make sig_extra in TcSigInfo a (Maybe SrcSpan)

Previously, sig_extra was a Boolean indicating the presence of an
extra-constraints wildcard. Now we store a (Maybe SrcSpan). We store the
location iff an extra-constraints wildcard is present. We'll need this SrcSpan
later to have an accurate location when we inform the user of the extra
constraints the wildcard generated.
No duplicates 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