rust-cpython: move $leaked struct out of macro
It wasn't easy to hack the $leaked struct since errors in macro would
generate lots of compile errors. Let's make it a plain struct so we can
easily extend it.
PyLeakedRef keeps a more generic PyObject instead of the $name struct
since it no longer has to call any specific methods implemented by
the $name class. $leaked parameter in py_shared_iterator!() is kept
for future change.
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors