gh-158585: Optimize bytes object creation

[vstinner]

Add bytes_alloc() helper function. bytes_alloc() and _PyBytes_FromStringAndSize() have less conditional branches than previous code, and so should be a little bit faster.

• Rename existing bytes_alloc() to bytes_type_alloc().
• Replace _PyBytes_FromSize(calloc=1) with _PyBytes_FromSizeZero().
• Add set_ob_shash_unsafe(): similar to set_ob_shash() but don't use an atomic operation on Free Threading. Use this new function on newly allocated bytes objects and in bytes_resize_inplace().
• Replace PyBytes_FromStringAndSize(NULL, size) with bytes_alloc(size).
• Use bytes_alloc() in PyBytes_FromString() and _PyBytes_Repeat().
• Add _PyBytes_FromStringAndSize(): similar to PyBytes_FromStringAndSize(), but str must not be NULL.
• Replace PyBytes_FromStringAndSize() with _PyBytes_FromStringAndSize().
• Test that PyBytes_FromString() and PyBytes_FromStringAndSize() return singletons for 0 or 1 bytes.

• Issue: C API: Rework and enhance the PyBytesWriter implementation #158585

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Results of PR gh-158665 benchmark: Mean +- std dev: [ref] 34.8 ns +- 0.4 ns -> [change] 32.2 ns +- 0.5 ns: 1.08x faster.

The change makes PyBytesWriter_Finish() 1.08x faster, it saves 2.6 nanoseconds.

[vstinner]

Oh. A side effect of this change is that b'abc' * 3 now returns the empty string singleton. I added a test for that.

[vstinner]

I also ran a benchmark on PyBytes_FromStringAndSize("abc", 3) and PyBytes_FromString("abc"): there is no significant different on performance.

        PyObject *bytes = PyBytes_FromStringAndSize("abc", 3);
        if (bytes == NULL) {
            return NULL;
        }
        Py_DECREF(bytes);

and:

        PyObject *bytes = PyBytes_FromString("abc");
        if (bytes == NULL) {
            return NULL;
        }
        Py_DECREF(bytes);