script: Use enums for event loop senders and receivers (#34896)

Previously, senders and receivers to different kinds of event loops (the
main `ScriptThread`, different types of workers) used a rust `trait`
mechanism to implement dynamic behavior. This led to having many unused
implementations of this `trait`. This change moves to using an `enum`
based approach for these senders and receivers and removes all of the
dead code.

In addition, to allowing for use of rust's dead code detection, it
simplifies the code a great deal. All of these generic senders and
receivers are moved to the `messaging.rs` file and given proper
documentation.

Finally, empty an `JSTraceable` implementation is made for all
crossbeam `Sender<...>`s to avoid having to manually skip them everytime
they are included in structs. The pre-existing empty `MallocSizeOf`
implementation is used more thoroughly.

Other unecessary wrappers around these senders and receivers are removed
as well.

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

components/script/dom/abstractworker.rs

@@ -7,7 +7,7 @@ use servo_url::ImmutableOrigin;
 
 use crate::dom::bindings::refcounted::Trusted;
 use crate::dom::bindings::reflector::DomObject;
-use crate::script_runtime::CommonScriptMsg;
+use crate::messaging::CommonScriptMsg;
 
 /// Messages used to control the worker event loops
 pub enum WorkerScriptMsg {