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/script/dom/history.rs

@@ -6,8 +6,8 @@ use std::cell::Cell;
 use std::cmp::Ordering;
 
 use base::id::HistoryStateId;
+use constellation_traits::TraversalDirection;
 use dom_struct::dom_struct;
-use embedder_traits::TraversalDirection;
 use js::jsapi::Heap;
 use js::jsval::{JSVal, NullValue, UndefinedValue};
 use js::rust::{HandleValue, MutableHandleValue};