Allow prototype references to be actual references

[steamroller-airmash]

This commit is a rather big design change in how prototypes are written and used. Before this, references to other prototypes were just strings. If you wanted to get the prototype that it was referring to it was necessary to go and look it up in the hashmaps contained within GameConfig. This was doable, if inconvenient.

It would be much more convenient if, instead of going through strings and hashmaps, we could just directly access the corresponding prototype to do this. This commit makes it so this approach works.

The way it does this is like so:

• Prototypes are parameterized by a type implementing the trait PrototypeRef. This is a sealed trait which provides a few associated types which are the types of each individual prototype reference.
• There are two concrete implementors of PrototypeRef: StringRef and PtrRef. These set the prototype references to be strings and actual references, respectively.
• Finally, when we convert between GamePrototype and GameConfig the string references are converted into real references and the whole thing is leaked so they all have a static lifetime.

The disadvantages of this approach is that the types are rather messy and that it leaks memory every time a GameConfig instance is constructed. However, the amount of memory is rather small and GameConfig should only be constructed about once for each execution of the program, so I think this downside is worth it.

In addition, GameConfig has an unsafe method to reclaim the leaked memory and actually free it. Usually there's no point in using this but it might be needed if we want to track down other memory leaks in the program.