Fix Binary_Add for Python Strings

[daytonjallen]

We're currently including the addition of strings in our Number Protocol call PyNumber_Add, however Python has a separate way of adding strings.

From their handling of the Binary_Add bytecode:

// code ...
if (PyUnicode_CheckExact(left) &&
         PyUnicode_CheckExact(right)) {
    sum = unicode_concatenate(tstate, left, right, f, next_instr);
    /* unicode_concatenate consumed the ref to left */
}
else {
    sum = PyNumber_Add(left, right);
    Py_DECREF(left);
}
// ... code

[ALavallee]

Make sure it works for add on all others objects.

we might change something CPython if string still doesn't after.

[ALavallee]

We're not going to take any actions on that one. Unclear why CPython does it that way. It's probably faster for a very string intensive program but still.