rustdoc: add "src" links to individual impls

Since these impls can be scattered around quite a bit, it is nice to be able to jump to the location where individual methods and trait impls are defined.

NOTE: this needs an update to the CSS, which I'd like to leave for whoever is the "rustdoc frontend champion". The new [src] links are currently too large and bold. Also, the interaction with the "since version X" annotations is not good.

Fixes: #30416