Non-expressions can't be used on LHS of infix macro when in expression position.

Created by: natefaubion

var a = for <<!

Where <<! is some infix macro. There aren't a lot of use cases for it, but it does speak to the larger limitations of expression position infix macros. I only really noticed when debugging why multi parameter arrow lambas weren't working with 0.6.0. The reason being that (a, b, c) used to be an "expression" and so worked on the LHS, but due to #326 (closed), it no longer accepts it as an expression, and is thus not allowed in expression position.