merge es6.parameters.rest and es6.parameters.default transformers

This is necessary in order to retain correct function arity and to have
completely correct semantics. Sometimes features are tied together so much
that they would require so much desugaring to retain the correct semantics
that they'd be equivalent to... the normal transpiled output.

test/core/fixtures/transformation/es6.parameters/rest-nested-iife/actual.js

@@ -0,0 +1,6 @@
+function broken(x, ...foo) {
+  if (true) {
+    class Foo extends Bar { }
+    return hello(...foo)
+  }
+}

test/core/fixtures/transformation/es6.parameters/rest-nested-iife/expected.js

@@ -0,0 +1,35 @@
+"use strict";
+
+var _get = function get(_x, _x2, _x3) { var _again = true; _function: while (_again) { var object = _x, property = _x2, receiver = _x3; desc = parent = getter = undefined; _again = false; if (object === null) object = Function.prototype; var desc = Object.getOwnPropertyDescriptor(object, property); if (desc === undefined) { var parent = Object.getPrototypeOf(object); if (parent === null) { return undefined; } else { _x = parent; _x2 = property; _x3 = receiver; _again = true; continue _function; } } else if ("value" in desc) { return desc.value; } else { var getter = desc.get; if (getter === undefined) { return undefined; } return getter.call(receiver); } } };
+
+function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }
+
+function _inherits(subClass, superClass) { if (typeof superClass !== "function" && superClass !== null) { throw new TypeError("Super expression must either be null or a function, not " + typeof superClass); } subClass.prototype = Object.create(superClass && superClass.prototype, { constructor: { value: subClass, enumerable: false, writable: true, configurable: true } }); if (superClass) subClass.__proto__ = superClass; }
+
+function broken(x) {
+  for (var _len = arguments.length, foo = Array(_len > 1 ? _len - 1 : 0), _key = 1; _key < _len; _key++) {
+    foo[_key - 1] = arguments[_key];
+  }
+
+  if (true) {
+    var _ret = (function () {
+      var Foo = (function (_Bar) {
+        function Foo() {
+          _classCallCheck(this, Foo);
+
+          _get(Object.getPrototypeOf(Foo.prototype), "constructor", this).apply(this, arguments);
+        }
+
+        _inherits(Foo, _Bar);
+
+        return Foo;
+      })(Bar);
+
+      return {
+        v: hello.apply(undefined, foo)
+      };
+    })();
+
+    if (typeof _ret === "object") return _ret.v;
+  }
+}