Make arrays implement Traversable and KeyedTraversable

This diff changes HHVM so that arrays are considered to implement the
Traversable and KeyedTraversable interfaces (which have no methods).

The idea is that these interfaces will be useful for parameter type
constraints for PHP code that wants to be compatible with both arrays and
collections (and possibly other objects that implement these interfaces).

hphp/compiler/expression/binary_op_expression.cpp

@@ -579,8 +579,15 @@ ExpressionPtr BinaryOpExpression::foldConst(AnalysisResultConstPtr ar) {
           result = v1 || v2; break;
         case T_LOGICAL_AND:
           result = v1 && v2; break;
-        case T_INSTANCEOF:
-          result = false; break;
+        case T_INSTANCEOF: {
+          if (v1.isArray() && v2.isString() &&
+              interface_supports_array(v2.getStringData())) {
+            result = true;
+            break;
+          }
+          result = false;
+          break;
+        }
         default:
           return ExpressionPtr();
       }