Concrete types

[encukou]

New API "layers" (#2, or a "C-API overhaul") should use concrete types, e.g. PyDictObject * rather than PyObject *.
Then we get type safety from the compiler, and we don't need type checking at runtime.

In the C-specific API, we'll want:

• Simple upcasts (PyDictObject → PyObject) -- this is just a cast, but let's wrap it for type-safety
• Fallible downcasts (PyObject → PyDictObject) -- a function that raises on bad type, and a function that
• Possibly, if API functions autogenerated, we can use C++ overloads & C11 _Generic macros. That would allow taking either PyObject (and possibly raise) or the concrete type (with no overead).

[zooba]

I'm -1 on this. I think it leads to massive API bloat and restricts our ability to innovate in the future.

I'm not saying our current system of runtime type checking is ideal, it could definitely use an overhaul, but I don't think pushing it into this layer is the right move.