Make the game playable

[Taneb]

• Errored out player snakes should repeat previous move
• Each tick, all snakes advance by their move
• A snake which turns out to be dead in the new state dies. Its head is removed, and all other segments turn to food.
• Each turn, have n(life snakes)-1 pieces of food.
• Spawn snakes as just the head

[46bit]

Sirpent-rust now essentially works, albeit with many defects to be addressed. Here's an update on these bullet points:

• Errored-out player snakes die immediately, being replaced with food before anything can collide with them.
• Each tick, all snakes advance by their move.
• A snake which turns out to be dead in the new state dies. All segments turn to food.
  • "Its head is removed" might still be helpful for having logically simple rules.
• The only food available comes from dead snakes. Food is never removed even when eaten - obviously a bug.
• Spawn snakes as just the head.

I'll try to fill out all defects I can think of as issues to start tidying up!

[46bit]

I believe sirpent-rust nearly works! However it needs a few things:

• [x] Player management. When Snakes die, players need evicting (or at least ignoring).
• [x] Players should be told CauseOfDeath. I'm rather happy to bring this sirpent-go concept back so neatly in https://github.com/sirpent-team/sirpent-rust/commit/15299159a4ceda855c46532b205377f7ce89f906.

[46bit]

I believe this is now playable! 😄 Albeit with much refactoring needed.