Haxe3 compatibility

I've made some changes here to get munit to compile and run with Haxe3 (latest SVN) and Neko 2.0 (latest SVN).  

Most of the changes are following on from information on this page: http://haxe.org/manual/haxe3

I've tried to keep each fix in a separate commit so you can easily see what's going on.

The change from `haxe.Stack` to `haxe.CallStack` and from `js.Lib.document` to `js.Browser.document` will not be compatible with Haxe 2.10, so it might be worth not merging this pull request until Haxe3 is closer to release.  (If desired, I could add some conditional compilation so that it will work for both).  The rest of the changes should be backwards compatible.