No support for infix operator definitions

[benl]

We don't support infix definitions like:

f ++ g = ...

All operators must be define in prefix style like:

(++) f g = ...

I'm not sure it's a good idea to support the infix definition style, because GHC had problems with it when they introduced bang patterns. For example:

f ! x = ...

It that supposed to be a definition of the operator (!) or, the definition of a function "f" which is strict in its argument?

We may well want to support bang patterns or something similar in the future, so we're avoiding infix definitions for now.