Rollup merge of #33382 - birkenfeld:issue-30416, r=alexcrichton
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.