ddnet: Introduce ddnet DemoWriter

[Patiga]

Also add --ddnet option to demo_read_write tool.

It automatically writes deltas and only full snapshots every 5 seconds.
Delta timings verified with a random demo.

[heinrich5991]

It's a bit sad that this is married to gamenet_ddnet. This means that you can only update gamenet_ddnet in lockstep with demo if you want to update one of them. It seems the intersections points are few: message parsing, snapshot object serialization, snapshot object lengths. Could these be replaced by not parsing messages, getting objects as integer arrays and passing in a function for that instead? I guess you did it for usability, but I'm not sure how the usability compares if you don't do it.

[Patiga]

If the dependency is a blocking concern for a future release on crates.io, then I'll fix it.
Alternatively, maybe gamenet_ddnet could be made less version braking? #[non_exhaustive] could be added to enums, but we are missing a Default implementation on structs to use it there.

I myself am only interested in DDNet right now, so tried to get that to work, so that others can skip that work.
And yea, I do appreciate simple interfaces. Without it being blocking, I currently lack motivation to make that change.

[heinrich5991]

Alternatively, maybe gamenet_ddnet could be made less version braking?

Probably not, no. The protocol is going to change in ways that are incompatible for this crate.

[Patiga]

In case of no blocking concerns, could this please get merged?