There are a few canvas2d-related dependencies that haven't updated, but they
only use euclid internally so that's not blocking landing the rest of the
changes.
Given the size of this patch, I think it's useful to get this landed as-is.
This new implementation of the session history keeps track of
a single tree of browsing contexts and pipelines which represents
the active entry of the session history and it keeps track of
diffs between adjacent entries. This allows use to traverse across
the joint session history by applying diffs to the active tree.
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.
