(compiler) Add support for mutable instance fields

These are represented by C++ fields in the generated native code, plain and simple. Note that only private fields are currently supported; we support the public keyword but errors will currently be thrown if you attempt to define a public field.

I am thinking about keeping it like this until I see a strong need/use case for it, since it breaks the fundamental idea of encapsulation. (Public static fields could be one notable exception, for example)