rustdoc: use src consistently over source in CSS/JS
The two terms have been used, inconsistently, in closely related spots like the `src/` directory vs `source-files.js`, and with things like `src-sidebar-toggle` vs the `source-sidebar`. This PR changes most use of `source` to `src` instead (except the localStorage configuration variables, which would be very complicated to migrate).
It also renames `.srclink` to `.src`. This is mostly aiming to cut out one of those many little peanut-butter bits of bloat, and is consistent with how other link classes are done (like how you have `a.mod` stylesheet rules, but there's also a `mod` class put on the body tag).
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