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.
This Root<T> relies on the concept of a stable trace object, which doesn't
move for the whole lifetime of the T value. For safety reasons, T must be 'static.
The only implementor of StableTraceObject for now is Dom<T>.
In a later PR, DomRoot<T> will become a type alias of Root<Dom<T>>,
where Root<T> will be able to handle all the things that need to be
rooted that have a stable traceable address that doesn't move for the
whole lifetime of the root. Stay tuned.