Remove m.prop + m.withAttr (#2317)

* Remove `m.prop` + `m.withAttr`

- For many uses, `m.withAttr` is *more* verbose than just directly using
  an event handler
- If you're using it with a bound callback, you're literally wasting a
  single character in the human readable version (and you're *saving*
  them in the minified output).
- It sometimes obscures your intent, if overused.
- Functions are easier to compress than `m.withAttr`, resulting in
  slightly smaller bundles.
- `m.withAttr` is overused anyways.
- `m.prop` is basically useless without `m.withAttr`, and the API
  doesn't have the same benefits it had with 0.2.x.

* Update changelog

index.js

@@ -13,8 +13,6 @@ requestService.setCompletionCallback(redrawService.redraw)
 
 m.mount = require("./mount")
 m.route = require("./route")
-m.withAttr = require("./util/withAttr")
-m.prop = require("./util/prop")
 m.render = require("./render").render
 m.redraw = redrawService.redraw
 m.request = requestService.request