Real classes

This is just a heads-up about some changes to the runtime. Before, a class was basically just a Proc instance, with some added extras and hacks to make it look different from a Proc. This was because under the hood, we used a Function and its prototype to store methods etc.

The problem with that, was that as classes were just a functions, they couldnt inherit methods (class methods), so everytime we add a class method, we would have to manually add those methods to subclasses.

Well, no longer!

This commit makes classes be real opal objects, so they are just literal instances of `Class`. There shouldnt really be any breaking changes here.