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/constellation/tracing.rs

@@ -46,7 +46,7 @@ mod from_compositor {
         };
     }
 
-    impl LogTarget for compositing_traits::ConstellationMsg {
+    impl LogTarget for constellation_traits::ConstellationMsg {
         fn log_target(&self) -> &'static str {
             match self {
                 Self::Exit => target!("Exit"),