This is part of the preparation for splitting stylo into a separate
crate. We have made various changes to selectors includings:
1. Bumping the rust edition
2. Fixing typos and updating links
In addition to reverting those changes, this PR pulls in some changes to
selectors we seem to have missed in the process of updates.
Fixes warnings from rust-lang/rust#44229 when `--enable-commonmark` is
passed to rustdoc.
This is mostly a global find-and-replace for bare URIs on lines by
themselves in doc comments.