Create a constellation_traits crate (#36088)

This change creates a `constellation_traits` crate. Previously messages
to the `Constellation` were in the `compositing_traits` crate, which
came about organically. This change moves these to a new crate which
also contains data types that are used in both compositing/libservo and
script (ie types that cross the process boundary). The idea is similar
to `embedding_traits`, but this is meant for types not exposed to the
API.

This change allows deduplicating `UntrustedNodeAddress`, which
previously had two versions to avoid circular dependencies.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/compositing/lib.rs

@@ -7,7 +7,8 @@
 use std::cell::Cell;
 use std::rc::Rc;
 
-use compositing_traits::{CompositorProxy, CompositorReceiver, ConstellationMsg};
+use compositing_traits::{CompositorProxy, CompositorReceiver};
+use constellation_traits::ConstellationMsg;
 use crossbeam_channel::Sender;
 use embedder_traits::ShutdownState;
 use profile_traits::{mem, time};