Derived Functor instance for void types

[David Feuer]

Currently, if you write

data V a deriving Functor

GHC generates

fmap _ _ = error "Void fmap"

This seems quite unfortunate, because it loses potentially useful error
information:

fmap (+ 3) (error "Too many snozzcumbers!")

throws "Void fmap", rather than the much more precise "Too many
snozzcumbers!" I've opened Trac #13117 to fix this, but I figured I should
double check that no one is opposed.

David Feuer
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