Add class-based object-oriented programming to Typed Lua

This project seeks to design and implement a class system for Typed Lua which will replace its current class definition style. It should feel like a cohesive component of the language, rather than a layer on top of Lua's meta-programming primitives. Because Typed Lua compiles to Lua and is designed to interact with regular Lua code, its code generator should support user-defined plug-ins for compiling Typed Lua class definitions into a specific untyped Lua class system.