[mypyc] feat: cache ids for fallback pythonic method lookups [1/1]

[BobTheBuidler]

This PR microoptimizes the usage of _Py_IDENTIFIER and _PyUnicode_FromId in various C files.

Changes:

• For each identifier (e.g., setdefault, update, keys, values, items, clear, copy), a static cache variable (e.g., setdefault_id_unicode) is used to store the result of _PyUnicode_FromId.
• The _Py_IDENTIFIER(name) macro is now declared only inside the conditional block that runs the first time a function is called (i.e., when the corresponding cache variable is NULL).
• This ensures that the interned unicode object is initialized only once, and only if needed.
• All subsequent calls reuse the cached unicode object, avoiding repeated calls to _PyUnicode_FromId and repeated static identifier declarations.

Example pattern after refactor:

static PyObject *setdefault_id_unicode = NULL;

PyObject *CPyDict_SetDefault(PyObject *dict, PyObject *key, PyObject *value) {
    if (PyDict_CheckExact(dict)) {
        PyObject* ret = PyDict_SetDefault(dict, key, value);
        Py_XINCREF(ret);
        return ret;
    }
    if (setdefault_id_unicode == NULL) {
        _Py_IDENTIFIER(setdefault);
        setdefault_id_unicode = _PyUnicode_FromId(&PyId_setdefault); /* borrowed */
        if (setdefault_id_unicode == NULL) {
            return NULL;
        }
    }
    return PyObject_CallMethodObjArgs(dict, setdefault_id_unicode, key, value, NULL);
}