rustdoc: Add stability and source links to the methods

Fixes #12932. This also fixes a span for the provided methods (originally excluded the method body), and provides an initial stylesheet for updated stability and source links ("better than nothing", adjustment and/or redesign welcomed). The rustdoc renders now look like this:

![](https://cloud.githubusercontent.com/assets/323836/2811899/c0d490bc-ce37-11e3-9a36-b927332cd1f7.png)