bpo-43693: Compute deref offsets in compiler (gh-25152)

Merges locals and cells into a single array.
Saves a pointer in the interpreter and means that we don't need the LOAD_CLOSURE opcode any more

https://bugs.python.org/issue43693

Objects/codeobject.c

@@ -1547,8 +1547,6 @@ error:
 code._varname_from_oparg
 
     oparg: int
-    *
-    cell: bool = False
 
 (internal-only) Return the local variable name for the given oparg.
 
@@ -1556,12 +1554,9 @@ WARNING: this method is for internal use only and may change or go away.
 [clinic start generated code]*/
 
 static PyObject *
-code__varname_from_oparg_impl(PyCodeObject *self, int oparg, int cell)
-/*[clinic end generated code: output=c7d39c9723692c8f input=2945bb291d3a3118]*/
+code__varname_from_oparg_impl(PyCodeObject *self, int oparg)
+/*[clinic end generated code: output=1fd1130413184206 input=c5fa3ee9bac7d4ca]*/
 {
-    if (cell) {
-        oparg += self->co_nlocals;
-    }
     PyObject *name = PyTuple_GetItem(self->co_localsplusnames, oparg);
     if (name == NULL) {
         return NULL;