bpo-43244: Remove the pyarena.h header (GH-25007)

Remove the pyarena.h header file with functions:

* PyArena_New()
* PyArena_Free()
* PyArena_Malloc()
* PyArena_AddPyObject()

These functions were undocumented, excluded from the limited C API,
and were only used internally by the compiler.

Add pycore_pyarena.h header. Rename functions:

* PyArena_New() => _PyArena_New()
* PyArena_Free() => _PyArena_Free()
* PyArena_Malloc() => _PyArena_Malloc()
* PyArena_AddPyObject() => _PyArena_AddPyObject()
diff --git a/Python/ast_opt.c b/Python/ast_opt.c
index 64fa067..0310466 100644
--- a/Python/ast_opt.c
+++ b/Python/ast_opt.c
@@ -16,7 +16,7 @@ make_const(expr_ty node, PyObject *val, PyArena *arena)
         PyErr_Clear();
         return 1;
     }
-    if (PyArena_AddPyObject(arena, val) < 0) {
+    if (_PyArena_AddPyObject(arena, val) < 0) {
         Py_DECREF(val);
         return 0;
     }