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.

api/tests/test-router.js

@@ -111,6 +111,25 @@ o.spec("route", function() {
 					o(view.callCount).equals(2)
 				})
 
+				o("subscribes correctly and removes when unmounted", function() {
+					$window.location.href = prefix + "/"
+
+					route(root, "/", {
+						"/" : {
+							view: function() {
+								return m("div")
+							}
+						}
+					})
+
+					o(root.firstChild.nodeName).equals("DIV")
+
+					// unsubscribe as if via `m.mount(root)`
+					redrawService.unsubscribe(root)
+
+					o(root.childNodes.length).equals(0)
+				})
+
 				o("default route doesn't break back button", function(done) {
 					$window.location.href = "http://old.com"
 					$window.location.href = "http://new.com"