Environment Refinements

This update adds a few new features to the environment API to make it more flexible and offer a bit more depth.

• Environment Heat and Frost Resistance can now accept negative values. A negative environment resistance now provides a random chance of doubling a temperature decrease from an environmental source.
• Added a priority field to the environment definition. This is used to sort the order that environment providers are applied to a biome during a lookup.
• Implementation detail only: environment providers are now stored as a field of the Biome class, this should allow for slightly faster lookup times.