[proposal] callback on deserializing entities

[sebovzeoueb]

As an initial step towards making deserialization more modular, I think it would be useful to have an optional callback that would execute on deserializing entities to give access to the EIDs present in the packet.

deserialize(world, packet, eid => {
    // do cool stuff with EID here
})

[sebovzeoueb]

latest resolves this in a more elegant way