bpo-46541: Replace core use of _Py_IDENTIFIER() with statically initialized global objects. (gh-30928)

We're no longer using _Py_IDENTIFIER() (or _Py_static_string()) in any core CPython code.  It is still used in a number of non-builtin stdlib modules.

The replacement is: PyUnicodeObject (not pointer) fields under _PyRuntimeState, statically initialized as part of _PyRuntime.  A new _Py_GET_GLOBAL_IDENTIFIER() macro facilitates lookup of the fields (along with _Py_GET_GLOBAL_STRING() for non-identifier strings).

https://bugs.python.org/issue46541#msg411799 explains the rationale for this change.

The core of the change is in:

* (new) Include/internal/pycore_global_strings.h - the declarations for the global strings, along with the macros
* Include/internal/pycore_runtime_init.h - added the static initializers for the global strings
* Include/internal/pycore_global_objects.h - where the struct in pycore_global_strings.h is hooked into _PyRuntimeState
* Tools/scripts/generate_global_objects.py - added generation of the global string declarations and static initializers

I've also added a --check flag to generate_global_objects.py (along with make check-global-objects) to check for unused global strings.  That check is added to the PR CI config.

The remainder of this change updates the core code to use _Py_GET_GLOBAL_IDENTIFIER() instead of _Py_IDENTIFIER() and the related _Py*Id functions (likewise for _Py_GET_GLOBAL_STRING() instead of _Py_static_string()).  This includes adding a few functions where there wasn't already an alternative to _Py*Id(), replacing the _Py_Identifier * parameter with PyObject *.

The following are not changed (yet):

* stop using _Py_IDENTIFIER() in the stdlib modules
* (maybe) get rid of _Py_IDENTIFIER(), etc. entirely -- this may not be doable as at least one package on PyPI using this (private) API
* (maybe) intern the strings during runtime init

https://bugs.python.org/issue46541

Objects/unicodeobject.c

@@ -260,11 +260,7 @@ get_unicode_state(void)
 // Return a borrowed reference to the empty string singleton.
 static inline PyObject* unicode_get_empty(void)
 {
-    struct _Py_unicode_state *state = get_unicode_state();
-    // unicode_get_empty() must not be called before _PyUnicode_Init()
-    // or after _PyUnicode_Fini()
-    assert(state->empty_string != NULL);
-    return state->empty_string;
+    return &_Py_STR(empty);
 }
 
 
@@ -1388,25 +1384,6 @@ _PyUnicode_Dump(PyObject *op)
 }
 #endif
 
-static int
-unicode_create_empty_string_singleton(struct _Py_unicode_state *state)
-{
-    // Use size=1 rather than size=0, so PyUnicode_New(0, maxchar) can be
-    // optimized to always use state->empty_string without having to check if
-    // it is NULL or not.
-    PyObject *empty = PyUnicode_New(1, 0);
-    if (empty == NULL) {
-        return -1;
-    }
-    PyUnicode_1BYTE_DATA(empty)[0] = 0;
-    _PyUnicode_LENGTH(empty) = 0;
-    assert(_PyUnicode_CheckConsistency(empty, 1));
-
-    assert(state->empty_string == NULL);
-    state->empty_string = empty;
-    return 0;
-}
-
 
 PyObject *
 PyUnicode_New(Py_ssize_t size, Py_UCS4 maxchar)
@@ -2009,10 +1986,11 @@ unicode_dealloc(PyObject *unicode)
 static int
 unicode_is_singleton(PyObject *unicode)
 {
-    struct _Py_unicode_state *state = get_unicode_state();
-    if (unicode == state->empty_string) {
+    if (unicode == &_Py_STR(empty)) {
         return 1;
     }
+
+    struct _Py_unicode_state *state = get_unicode_state();
     PyASCIIObject *ascii = (PyASCIIObject *)unicode;
     if (ascii->state.kind != PyUnicode_WCHAR_KIND && ascii->length == 1) {
         Py_UCS4 ch = PyUnicode_READ_CHAR(unicode, 0);
@@ -15551,11 +15529,14 @@ _PyUnicode_InitState(PyInterpreterState *interp)
 PyStatus
 _PyUnicode_InitGlobalObjects(PyInterpreterState *interp)
 {
-    struct _Py_unicode_state *state = &interp->unicode;
-    if (unicode_create_empty_string_singleton(state) < 0) {
-        return _PyStatus_NO_MEMORY();
+    if (!_Py_IsMainInterpreter(interp)) {
+        return _PyStatus_OK();
     }
 
+#ifdef Py_DEBUG
+    assert(_PyUnicode_CheckConsistency(&_Py_STR(empty), 1));
+#endif
+
     return _PyStatus_OK();
 }
 
@@ -15798,15 +15779,14 @@ PyDoc_STRVAR(length_hint_doc, "Private method returning an estimate of len(list(
 static PyObject *
 unicodeiter_reduce(unicodeiterobject *it, PyObject *Py_UNUSED(ignored))
 {
-    _Py_IDENTIFIER(iter);
     if (it->it_seq != NULL) {
-        return Py_BuildValue("N(O)n", _PyEval_GetBuiltinId(&PyId_iter),
+        return Py_BuildValue("N(O)n", _PyEval_GetBuiltin(&_Py_ID(iter)),
                              it->it_seq, it->it_index);
     } else {
         PyObject *u = (PyObject *)_PyUnicode_New(0);
         if (u == NULL)
             return NULL;
-        return Py_BuildValue("N(N)", _PyEval_GetBuiltinId(&PyId_iter), u);
+        return Py_BuildValue("N(N)", _PyEval_GetBuiltin(&_Py_ID(iter)), u);
     }
 }
 
@@ -16137,7 +16117,6 @@ _PyUnicode_Fini(PyInterpreterState *interp)
     for (Py_ssize_t i = 0; i < 256; i++) {
         Py_CLEAR(state->latin1[i]);
     }
-    Py_CLEAR(state->empty_string);
 }