Extend 'Default' hints to complain when people reinvent the Either a fun...

...ctor

I just noticed that I accidentally reimplemented the 'Either a' functor
in my code, having

  compile = either Left (Right . optimize) . parse

Let hlint detect this case, suggesting to just use

  compile = fmap optimize . parse

instead; the Functor instance for 'Either a' does just what we want.