Implement survival mode

This PR implements survival mode. Resolves #40.

#213 is blocked on this as we are developing the design we will use to approach gamemodes, and moreover, how we will implement gameplay features with regard to component architecture. The current approach is heavily based on data-oriented design. Rather than checking a player's gamemode in systems, there are a number of marker components—e.g. CanBreak, CanInstaBreak, CanTakeDamage, CanFly—which describe the capabilities of a player. These are inserted based on gamemode on player join, but the architecture allows for extensive customization of functionality through plugins.

Ideally, we would have a /gamemode command, but this is blocked on #209 being merged. I may merge it early, depending on how complete people deem it to be. Currently, the only way to affect a player's gamemode is through the default-gamemode setting in the config or manual editing of the player data files.

# Inventory handling
Only basic survival mode inventory handling (i.e. single clicks) has been implemented. I will hold off on the rest until our inventory indexing situation has improved.

# Block breaking
Survival mode digging has been implemented, for the most part. All which remains is to write down the harvest tools for all blocks (currently, only stone and dirt are in the list). This file is in definitions/data/tool.ron.