Add new functions to pack and unpack C double (serialize and deserialize):
:c:func:`PyFloat_Pack2`, :c:func:`PyFloat_Pack4`, :c:func:`PyFloat_Pack8`,
:c:func:`PyFloat_Unpack2`, :c:func:`PyFloat_Unpack4` and
:c:func:`PyFloat_Unpack8`. Patch by Victor Stinner.
