convert AST_Seq from binary tree to array (#1460)

- rename `AST_Seq` to `AST_Sequence`
- raise default sequences_limit from 200 to 800

lib/parse.js

@@ -1527,17 +1527,18 @@ function parse($TEXT, options) {
 
     var expression = function(commas, no_in) {
         var start = S.token;
-        var expr = maybe_assign(no_in);
-        if (commas && is("punc", ",")) {
+        var exprs = [];
+        while (true) {
+            exprs.push(maybe_assign(no_in));
+            if (!commas || !is("punc", ",")) break;
             next();
-            return new AST_Seq({
-                start  : start,
-                car    : expr,
-                cdr    : expression(true, no_in),
-                end    : peek()
-            });
+            commas = true;
         }
-        return expr;
+        return exprs.length == 1 ? exprs[0] : new AST_Sequence({
+            start       : start,
+            expressions : exprs,
+            end         : peek()
+        });
     };
 
     function in_loop(cont) {