This is based on compiling with `RUSTFLAGS="-W unused_crate_dependencies"` (CC https://github.com/rust-lang/rust/pull/72342) in a recent Nightly (more so than used in the tree as of this writing, CC https://github.com/servo/servo/issues/26661 for work-arounds).
Only one crate is actually removed from the dependency graph, others are still dependended from other places.
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.