more optimizations that v1 does and some cleanups

- a = a + x ==> a+=x
- joining consecutive var statements (hoisting is not always desirable)
- x == false ==> x == 0, x != true ==> x != 1
- x, x ==> x; x = exp(), x ==> x = exp()
- discarding useless break-s

lib/parse.js

@@ -1455,8 +1455,8 @@ function parse($TEXT, exigent_mode) {
             next();
             return new AST_Seq({
                 start  : start,
-                first  : expr,
-                second : expression(true, no_in),
+                car    : expr,
+                cdr    : expression(true, no_in),
                 end    : peek()
             });
         }