Deduplicate m.route and m.redraw logic (#2453)

- Remove appropriate route change subcriptions when a root is removed
  via `m.mount(root, null)`.
- Don't pollute `onpopstate` and friends - use standard event listeners
  instead.
- Simplify and streamline subscriptions, in preparation of adding a
  `remove` parameter to `m.mount`.
- Change the redraw internals to redraw immediately, with ability to
  cancel via returning a sentinel.
- Change `"bleeding-edge"` for `m.version` in `next` to instead just be
  the latest `m.version`. (If you're using `next`, you should know what
  you're in for.)
- Update tests to be aware of these changes. (Some were failing for
  subtle reasons.)
- Drive-by: remove some uses of `string.charAt(n)` and use `string[n]`
  instead.

test-utils/pushStateMock.js

@@ -187,5 +187,13 @@ module.exports = function(options) {
 	$window.onhashchange = null,
 	$window.onunload = null
 
+	$window.addEventListener = function (name, handler) {
+		$window["on" + name] = handler
+	}
+
+	$window.removeEventListener = function (name, handler) {
+		$window["on" + name] = handler
+	}
+
 	return $window
 }