small extension to `...` notation

[Doaitse Swierstra]

In Haskell we write `f` in order to infixify the identifier f. In ABC  
the stuff between backquotes is not limited to an identifier, but any  
expression may occur there. This would allow one to write e.g.

  xs `zipWith (+)` ys

In general  <expr1> `<expr2>` <expr3> => (<expr2>) <expr1> <expr3>

I think it is a small extension to Haskell, which does not break  
anything existing, and provides yet another opportunity to beautify  
one programs, especially in combination with programs like lhs2TeX.

  Doaitse Swierstra