React inlining: Refactor to reduce parsing cost

- Have the `jsx` helper do the `defaultProps` work instead of calling `defaultProps` inline.
- Put `key` after `props` and make it optional.
- Inline `children` as rest args instead of in the object.
- Rename `createRawReactElement` to `jsx`. I wish I was kidding.

Most of these are silly microoptimizations. In my test file (based off an internal RN app), this reduces the parsing overhead of inlining from around 1% to 0.1% in JSC and from 0.6% to 0.0% in V8 (compared to element inlining before this commit).

Once parsed, the initial render with inlining is the same speed as not inlining in JSC and ~1% slower in V8. A second initial render in the same context (reusing the function objects, JIT, etc) is 2.0% faster in JSC and 5.5% faster in V8.

packages/babel-plugin-transform-react-inline-elements/test/fixtures/inline-elements/nested/expected.js

@@ -1,4 +1,3 @@
-babelHelpers.createRawReactElement("div", null, {
-  className: "foo",
-  children: [bar, babelHelpers.createRawReactElement(Baz, "baz", Baz.defaultProps || {})]
-});
+babelHelpers.jsx("div", {
+  className: "foo"
+}, void 0, bar, babelHelpers.jsx(Baz, {}, "baz"));