Implement compact dict

Issue #27350: `dict` implementation is changed like PyPy. It is more compact
and preserves insertion order.

_PyDict_Dummy() function has been removed.

Disable test_gdb: python-gdb.py is not updated yet to the new structure of
compact dictionaries (issue #28023).

Patch written by INADA Naoki.

Include/object.h

@@ -710,7 +710,6 @@ you can count such references to the type object.)
 PyAPI_DATA(Py_ssize_t) _Py_RefTotal;
 PyAPI_FUNC(void) _Py_NegativeRefcount(const char *fname,
                                             int lineno, PyObject *op);
-PyAPI_FUNC(PyObject *) _PyDict_Dummy(void);
 PyAPI_FUNC(Py_ssize_t) _Py_GetRefTotal(void);
 #define _Py_INC_REFTOTAL        _Py_RefTotal++
 #define _Py_DEC_REFTOTAL        _Py_RefTotal--