Error out when seeing non-default fixity declarations

[bergmark]

Fay just passes base fixities to haskell-src-exts so these are the only operators that will parse correctly. Custom fixity declarations are ignored.

If you want to use custom operators anyway, be aware of the default fixity it gets and either declare it as such or remove the fixity declaration alltogether.

We can error if a non default fixity is given, otherwise compile normally.