Destructuring consistency fixes (#1417)

- Use AST_Destructuring for lhf assignment patterns
- Use AST_DefaultAssign for default assignments
- Add more checks for lhs expressions
- Add lots of testing
- Cleanup ast (e.g. remove default property)
- Fix #1402 based on a patch from @kzc
- Refine spread allowance in array destructring pattern
- Add destructuring AST tree checker

test/compress/issue-203.js

@@ -13,7 +13,11 @@ compress_new_function: {
 
 compress_new_function_with_destruct: {
     options = {
-        unsafe: true
+        unsafe: true,
+        ecma: 6
+    }
+    beautify = {
+        ecma: 6
     }
     input: {
         new Function("aa, [bb]", 'return aa;');