Fix name clash for global leaks when using --readable-names

Created by: natefaubion

This is a bit of an obscure bug, but can come up if you're using --readable-names and messing around with hygiene. Normally, it would result in a ReferenceError or something at runtime, but --readable-names could mask the issue, resulting in hard to track down bugs.

Take this code:

macro clobber {
  case { _ $body } => {
    var b = #{ $body };
    b[0].context = null;
    return b;
  }
}

var a = 12;
function foo() {
  var a = 1;
  var b = clobber a;
  return b;
}

Normally, this would be compiled as such:

var a$281 = 12;
function foo$282() {
    var a$283 = 1;
    var b$285 = a;
    return b$285;
}

Notice the a reference doesn't have a hygiene tag because we clobbered it's context. With --readable-names this would get expanded to:

var a = 12;
function foo() {
    var a$2 = 1;
    var b = a;
    return b;
}

Notice how the a in b = a would refer to the outer a? This PR splits --readable-names into two passes on the AST: one to gather all the non-static/global reference, and one to mangle. It will now expand correctly to:

var a$2 = 12;
function foo() {
    var a$3 = 1;
    var b = a;
    return b;
}

Preserving the same hygiene as the expansion without --readable-names.