v0.2.0 - split server/client types

[simbleau]

This change is primarily motivated by Bevy's ecosystem standards - "features are additive".

If someone uses both the server and client feature in v0.1, they'd inherit two different types of NetworkReader and NetworkWriter, one for a client, one for a server.

The changes in this PR allow someone to use both features without name collision. The NetworkReader and NetworkReader have been combined into a single type, now respectively RtcClient or RtcServer.

In addition, this PR also solves a thorny borrow issue.
Now that NetworkReader and NetworkWriter have been combined, the following was not possible on clients:

for .. in client.read() {
    client.reliable_to_host(..)
}

This is because .read() takes &mut self and so does .reliable_to_host(). (2 mutable borrows)

A minor change to address this was that the definition of read() changed on the client to return an owned Vec<M> instead of a VecDeque<'_, M>. The lifetime was problematic.