Allow seamless entity mapping

[cBournhonesque]

Currently we only do entity mapping when we deserialize a message, by looking at the remote_to_local map.

We didn't do the reverse, which meant that sending a message referring to a local entity on the entity-receiver side was annoying on because the remote is the entity-sender so it wouldn't apply entity mapping.

Now we also apply entity mapping when we serialize a message, by using the local_to_remote map.

I didn't implement this previously because of potential conflicts but I think that the risk of those is pretty small. It would require a user to have both client->server and server->client in the same game.
In the long-term I want to solve those conflicts by using pre-defined ID ranges.
In the meantime we can greatly improve ergonomics by enabling this feature.

The conflicts could be for example:

• Server spawns E1 and replicates to client who spawns E2. Client has E1 remote -> E2 local mapping.
• Client spawns E1 and replicates to server who spawns E3, Server has E1 remote -> E3 local mapping.
  (basically the conflict happens if the server and client both spawn and replicate an entity with the same Entity)

Client sends a message containing E2.
It maps it from local to remote: E1
Server receives the message with E1, it maps it from remote_to_local -> E3. (instead of just using the original E1)

Solutions to avoid these kinds of conflicts:

• avoid replicating in both directions
• predefined entity ranges for networking
• include a bit in the message that indicates that mapping was already done. If the mapping was already done on the sender, we don't do it again on the receiver. Maybe that bit can be included directly in the Entity, similar to what's described in https://ajmmertens.medium.com/doing-a-lot-with-a-little-ecs-identifiers-25a72bd2647
  On the receiving side we would check this bit to know it entity-mapping has already been applied or not?
  (or just 1 bit per message, but that seems expensive)