Issue #29300: Convert _struct module to Argument Clinic

* The struct module now requires contiguous buffers.
* Convert most functions and methods of the _struct module to Argument Clinic
* Use "Py_buffer" type for the "buffer" argument. Argument Clinic is
  responsible to create and release the Py_buffer object.
* Use "PyStructObject *" type for self to avoid explicit conversions.
* Add an unit test on the _struct.Struct.unpack_from() method to test passing
  arguments as keywords.
* Rephrase docstrings.
* Rename "fmt" argument to "format" in docstrings and the documentation.

As a side effect, functions and methods which used METH_VARARGS calling
convention like struct.pack() now use the METH_FASTCALL calling convention
which avoids the creation of temporary tuple to pass positional arguments and
so is faster. For example, struct.pack("i", 1) becomes 1.56x faster (-36%)::

    $ ./python -m perf timeit \
        -s 'import struct; pack=struct.pack' 'pack("i", 1)' \
        --compare-to=../default-ref/python
    Median +- std dev: 119 ns +- 1 ns -> 76.8 ns +- 0.4 ns: 1.56x faster (-36%)
    Significant (t=295.91)

Patch co-written with Serhiy Storchaka.

Lib/test/test_struct.py

@@ -412,6 +412,10 @@ class StructTest(unittest.TestCase):
             for i in range(6, len(test_string) + 1):
                 self.assertRaises(struct.error, struct.unpack_from, fmt, data, i)
 
+        # keyword arguments
+        self.assertEqual(s.unpack_from(buffer=test_string, offset=2),
+                         (b'cd01',))
+
     def test_pack_into(self):
         test_string = b'Reykjavik rocks, eow!'
         writable_buf = array.array('b', b' '*100)