Turn all cdef class objects in the Python layer to cdef public class

[leofang]

This allows us to share the class declarations with C/C++:
https://cython.readthedocs.io/en/latest/src/userguide/external_C_code.html#using-cython-declarations-from-c
which is needed after PR #463 set up a path for Python-C/C++/Cython interoperability, but without adding the public qualifier the interoperability was limited to Cython only.

[leofang]

@vzhurba01 I hope this is a simple one-liner change to the codegen and the remaining task is on verifying the generated headers. Could you take a look?