Consider implications of payload system

[zesterer]

As of a recent commit, the client moved to a payload system for frontend wishing to add additional payloads of their own to types used inside the client. Currently, we only have a single associated type, but this is ripe for expanding to other areas of the codebase: https://github.com/veloren/game/blob/master/client/src/lib.rs#L60.

We need to think about:

• Is this a good idea?

• What implications may it have down the line?

• Is it worth expanding this to the server and other areas of the codebase?

• How many other uses does it have?

• Does it improve or hurt readability?

[veloren-bot]

moved to gitlab https://gitlab.com/veloren/game/issues/16