Generic Types

[sunjay]

Depends on #16 before this can be implemented. Probably need #44 before this is particularly useful.

• [ ] allow definition of generic type parameters in functions, structs (#39), enums (#41), etc.
• [ ] generic type bounds
• [ ] Remove ItemType::Any and replace any usages with type bounds
• [ ] Make sure types work at declaration and don't have to be ordered in a specific way (you can order types in any order in the file)
• [ ] Defaults for generic types