gh-113317: Argument Clinic: Add libclinic.return_converters (#117451)

Move the following converter classes to libclinic.return_converters:

* CReturnConverter
* CReturnConverterAutoRegister
* Py_ssize_t_return_converter
* bool_return_converter
* double_return_converter
* float_return_converter
* int_return_converter
* long_return_converter
* size_t_return_converter
* unsigned_int_return_converter
* unsigned_long_return_converter

Move also the add_c_return_converter() function there.