* avm2: Properly make all classes an instance of `Class`.
Also, does this technically mean that `Class` is a metaclass?
* avm2: Remove `Function::from_method_and_proto` as it will no longer be needed
* avm2: Ensure builtin classes are also instances of `Class`.
This requires tying a veritable gordian knot of classes; everything needs to be allocated up-front, linked together, and then properly initialized later on. This necessitated splitting the whole class construction process up into three steps:
1. Allocation via `from_class_partial`, which does everything that can be done without any other classes
2. Weaving via `link_prototype` and `link_type`, which links all of the allocated parts together correctly. This also includes initializing `SystemClasses` and `SystemPrototypes`.
3. Initialization via `into_finished_class`, which must be done *after* the weave has finished.
Once complete you have core classes that are all instances of `Class`, along with prototypes that have their usual legacy quirks.
Note that this does *not* make prototypes instances of their class. We do need to do that, but doing so breaks ES3 legacy support. This is because we currently only work with bound methods, but need to be able to call unbound methods in `callproperty`.
* tests: Add a test for all core classes' instance-of relationships
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