Let-defined punctuator macros parse inconsistently

Created by: dfreeman

Really happy to see a fix for #125 (closed) has gone in, but now that the let-defined punctuator macro definitions will parse, it looks like there are still some cases where their application is a problem.

As defined here, ^ behaves as the identity in all three cases:

macro ^ { rule { $x } => { $x } }

var a1 = ^ 1;           // => 1
var a2 = (^ 1);         // => 1
var a3 = { key: ^ 1 };  // => { key: 1 }

Defined with let, however, an error is thrown for the second two cases:

let ^ = macro { rule { $x } => { $x } }

var a1 = ^ 1;           // => 1
var a2 = (^ 1);         // => Error: Line 4: Unexpected token ^
var a3 = { key: ^ 1 };  // => Error: Line 5: Unexpected token ^

If the macro is given a regular JS identifier for a name, everything works as expected regardless of whether let is used.