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()

Python/symtable.c

@@ -1972,16 +1972,16 @@ _Py_SymtableStringObjectFlags(const char *str, PyObject *filename,
     mod_ty mod;
     PyArena *arena;
 
-    arena = PyArena_New();
+    arena = _PyArena_New();
     if (arena == NULL)
         return NULL;
 
     mod = _PyParser_ASTFromString(str, filename, start, flags, arena);
     if (mod == NULL) {
-        PyArena_Free(arena);
+        _PyArena_Free(arena);
         return NULL;
     }
     st = _PySymtable_Build(mod, filename, 0);
-    PyArena_Free(arena);
+    _PyArena_Free(arena);
     return st;
 }