Add the following functions for the new :class:`frozendict` type:

* :c:func:`PyAnyDict_Check`
* :c:func:`PyAnyDict_CheckExact`
* :c:func:`PyFrozenDict_Check`
* :c:func:`PyFrozenDict_CheckExact`
* :c:func:`PyFrozenDict_New`

Patch by Victor Stinner.
