Improve cursor movement and typing latency a bit

This PR combines a few small optimizations I found. It saves 1-2 ms per keystroke by:

* Avoiding invoking React's synthetic event handler for `keydown` and `textInput` events.
* Allowing `Emitter::emit` to be optimized by v8 by removing `args...` from the common path.
* Using raw DOM APIs instead of jQuery to receive keydown events
* Avoiding some temp objects in `atom-keymap`'s `keystrokeForKeyboardEvent` method.