request for reviews for my first patch -- ticket 7401

[Ömer Sinan Ağacan]

Removing `null data_cons`(compiler/typecheck/TcDeriv.lhs 1105) check
for eq instance declarations fixes this, code generation part would be
same(changes in TcGenDeriv.hs were wrong and TcGenDeriv needs to be
unchanged)but I'm not sure how to remove that check only for eq
instance(Show, Ord, etc. sould still fail with same error message and
they're checked by same function, `cond_stdOK` -- line 1053).

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Ömer Sinan Ağacan
http://osa1.net