Make all classes with the same instance size derive from a common base

In order to fully satisfy Python's inheritance type layout requirements,
all types should have a common 'solid' base. A solid base is one which
has the same instance size as the derived type (not counting the space
required for the optional `dict_ptr` and `weakrefs_ptr`). Thus, `object`
does not qualify as a solid base for pybind11 types and this can lead to
issues with multiple inheritance.

To get around this, new base types are created: one per unique instance
size. There is going to be very few of these bases. They ensure Python's
MRO checks will pass when multiple bases are involved.

tests/test_inheritance.cpp

@@ -36,6 +36,10 @@ public:
     Hamster(const std::string &name) : Pet(name, "rodent") {}
 };
 
+class Chimera : public Pet {
+    Chimera() : Pet("Kimmy", "chimera") {}
+};
+
 std::string pet_name_species(const Pet &pet) {
     return pet.name() + " is a " + pet.species();
 }
@@ -74,6 +78,8 @@ test_initializer inheritance([](py::module &m) {
     py::class_<Hamster, Pet>(m, "Hamster")
         .def(py::init<std::string>());
 
+    py::class_<Chimera, Pet>(m, "Chimera");
+
     m.def("pet_name_species", pet_name_species);
     m.def("dog_bark", dog_bark);