PEP 782: Add PyBytesWriter C API

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[AA-Turner]

Brief editorial review:

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@AA-Turner: I updated the PEP, please review it again.

[picnixz]

For the examples, it could help if you could add comments telling what happens if we were to call PyBytesWriter_Finish() after each call to a PyBytesWriter_* writing function. For instance in the hello world case:

PyObject* hello_world(void)
{
    PyBytesWriter *writer = PyBytesWriter_Create(0);
    if (writer == NULL) {
        goto error;
    }
    // PyBytesWriter_Finish(writer) == b""
    if (PyBytesWriter_WriteBytes(writer, "Hello", -1) < 0) {
        goto error;
    }
    // PyBytesWriter_Finish(writer) == b"Hello"
    if (PyBytesWriter_Format(writer, " %s!", "World") < 0) {
        goto error;
    }
    // PyBytesWriter_Finish(writer) == b"Hello World!"
    return PyBytesWriter_Finish(writer);
error:
    PyBytesWriter_Discard(writer);
    return NULL;
}