Differentiate single and broadcast message on type-system level (#166)

Prior to this patch-set, we had constructs like `MessageType` and `SentMessage`. These were used to write code that is generic over a particular message type. In reality however, we don't actually need to differentiate between these cases because the codepaths for each type are statically known in all cases.

Unfortunately, introducing an actual split introduced a bit more code (net ~ 70 lines) but in exchange, we remove a few `unreachable` error clauses and make it overall easier to follow the dataflow through xtra's channel implementation.

src/address.rs

@@ -12,7 +12,7 @@ use event_listener::EventListener;
 use futures_util::FutureExt;
 
 use crate::refcount::{Either, RefCounter, Strong, Weak};
-use crate::send_future::{Broadcast, ResolveToHandlerReturn};
+use crate::send_future::{ActorNamedBroadcasting, Broadcast, ResolveToHandlerReturn};
 use crate::{inbox, ActorNamedSending, Handler, SendFuture};
 
 /// An [`Address`] is a reference to an actor through which messages can be sent.
@@ -174,7 +174,7 @@ impl<A, Rc: RefCounter> Address<A, Rc> {
     ///
     /// The actor must implement [`Handler<Message>`] for this to work where [`Handler::Return`] is
     /// set to `()`.
-    pub fn broadcast<M>(&self, msg: M) -> SendFuture<ActorNamedSending<A, Rc>, Broadcast>
+    pub fn broadcast<M>(&self, msg: M) -> SendFuture<ActorNamedBroadcasting<A, Rc>, Broadcast>
     where
         M: Clone + Send + Sync + 'static,
         A: Handler<M, Return = ()>,

src/envelope.rs

@@ -8,7 +8,7 @@ use futures_core::future::BoxFuture;
 use futures_util::FutureExt;
 
 use crate::context::Context;
-use crate::inbox::{HasPriority, Priority};
+use crate::inbox::{HasPriority, MessageToAll, MessageToOne, Priority};
 use crate::{Actor, Handler};
 
 /// A message envelope is a struct that encapsulates a message and its return channel sender (if applicable).
@@ -170,7 +170,7 @@ impl<A, M, R> HasPriority for ReturningEnvelope<A, M, R> {
     }
 }
 
-impl<A> HasPriority for Box<dyn MessageEnvelope<Actor = A>> {
+impl<A> HasPriority for MessageToOne<A> {
     fn priority(&self) -> Priority {
         self.as_ref().priority()
     }
@@ -236,7 +236,7 @@ pub trait BroadcastEnvelope: HasPriority + Send + Sync {
     ) -> (BoxFuture<'a, ControlFlow<()>>, Span);
 }
 
-impl<A> HasPriority for Arc<dyn BroadcastEnvelope<Actor = A>> {
+impl<A> HasPriority for MessageToAll<A> {
     fn priority(&self) -> Priority {
         self.as_ref().priority()
     }