:expr doesn't allow function calls

This fails to match:

macro m {
  case ($x:expr) => {$x}
}
function id (x) { return x; }
var x = m( id(4) )

This is related to the outside-in/inside-out expansion stuff. Right now when trying to match the :expr the syntax is completely unexpanded but what we really need to do is a local-expand sort of thing.