gh-87613: Use AC VC for io.BytesIO (#160855)

BytesIO is commonly constructed in the test suite and gets significantly
faster to construct. Argument error messages are unchanged.

pyperf microbenchmark (PGO+LTO, run serially):

| Benchmark                    | base    | bytesio               |
|------------------------------|:-------:|:---------------------:|
| BytesIO(bytes)               | 54.2 ns | 27.0 ns: 2.01x faster |
| BytesIO(bytearray)           | 76.7 ns | 38.6 ns: 1.99x faster |
| BytesIO(initial_bytes=bytes) | 56.5 ns | 34.8 ns: 1.63x faster |
| BytesIO()                    | 40.9 ns | 25.4 ns: 1.61x faster |
| Geometric mean               | (ref)   | 1.48x faster          |

Benchmark hidden because not significant (2): `BytesIO.__new__`,
subclass(bytes)

Benchmark code:

```python
"""BytesIO() construction: AC @vectorcall vs tp_new + tp_init."""
import pyperf

runner = pyperf.Runner()

setup = "from io import BytesIO; b = b'abcdefgh'"
runner.timeit('BytesIO()', 'BytesIO()', setup)
runner.timeit('BytesIO(bytes)', 'BytesIO(b)', setup)
runner.timeit('BytesIO(bytearray)', 'BytesIO(x)', setup + '; x = bytearray(b)')
runner.timeit('BytesIO(initial_bytes=bytes)', 'BytesIO(initial_bytes=b)', setup)
runner.timeit('subclass(bytes)', 'S(b)', setup + '\nclass S(BytesIO): pass')
runner.timeit('BytesIO.__new__', 'new(BytesIO)',
              setup + '; new = BytesIO.__new__')
```

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: cmaloney

Modules/_io/bytesio.c

@@ -1118,6 +1118,7 @@ bytesio_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 }
 
 /*[clinic input]
+@vectorcall
 @critical_section
 _io.BytesIO.__init__
     initial_bytes as initvalue: object(c_default="NULL") = b''
@@ -1127,7 +1128,7 @@ Buffered I/O implementation using an in-memory bytes buffer.
 
 static int
 _io_BytesIO___init___impl(bytesio *self, PyObject *initvalue)
-/*[clinic end generated code: output=65c0c51e24c5b621 input=3da5a74ee4c4f1ac]*/
+/*[clinic end generated code: output=65c0c51e24c5b621 input=a6dc38082204df28]*/
 {
     if (FT_ATOMIC_LOAD_SSIZE_RELAXED(self->exports) > 0) {
         PyErr_SetString(PyExc_BufferError,
@@ -1256,6 +1257,7 @@ static PyType_Slot bytesio_slots[] = {
     {Py_tp_getset, bytesio_getsetlist},
     {Py_tp_init, _io_BytesIO___init__},
     {Py_tp_new, bytesio_new},
+    {Py_tp_vectorcall, _io_BytesIO_vectorcall},
     {0, NULL},
 };