Class parity for Carbide / Legacy

[simsaens]

Feature description

Carbide uses a more modern class system than Legacy (middleclass). We should continue to use the newer class system for Carbide but ensure that legacy classes still work without changes

At the moment I believe initialisation is not working for legacy classes in Carbide. That is, MyClass:init() is not called when constructing MyClass(). We can probably back-support this in Carbide.

[simsaens]

• init is called on Foo(...) in Carbide already, so constructor behavior is largely compatible.
• Missing/renamed APIs: is_a and _base are not present in Carbide’s middleclass.
• Inheritance semantics differ: Legacy copies base methods at subclass creation; middleclass propagates later changes to subclasses.
• Instances in Carbide get instance.class and __metaCodeaName, which Legacy never had.

We can probably ignore this issue unless it comes up in practice, in which case we could do:

• Add _base alias at class creation time and is_a alias in DefaultMixin.