[Agda] Mixfix binders: first patch

[Jean-Philippe Bernardy]

I have implemented a bit more of this, and uploaded the patch bundle
to the issue tracker.

You can now define simple binders. ie. a declaration like

syntax bind e1 (\x → e2) = x ← e1 ; e2

allows to write the rhs to mean the lhs.

http://code.google.com/p/agda/issues/detail?id=306

Cheers,
JP.