Cleanups and minor changes
Object update

-Rock surrounded by Fire no longer produces Lava
-Mixing is always enforced; Objects can define custom, post-mix update Streams
Flora update

-Added Dirt, produced by eroding Rock with Water
-Dirt spawns Grass to open areas
-Removed Grass spreading
-Removed a few keys
Placing Objects overwrites tiles instead of mixing
Object update overhaul

-Objects entirely define their updates (no enforced mixing)
-When an Object's Stream ends, the next Stream will be updated recursively
-Cycles in Objects updates take the earliest Object in the cycle
Smelt Rock into Lava, using the Rock and Fire keys
Volcano improvements
Merge Game.Input into Game.State
Cycles between Objects must be resolved explicitly