[Tolk] Support syntax tensorVar.0 and tupleVar.0

It works both for reading and writing:
> var t = (1, 2);
> t.0;      // 1
> t.0 = 5;
> t;        // (5, 2)

It also works for typed/untyped tuples, producing INDEX and SETINDEX.

Global tensors and tuples works. Nesting `t.0.1.2` works. `mutate` works.
Even mixing tuples inside tensors inside a global for writing works.

tolk/codegen.cpp

@@ -132,7 +132,7 @@ int Stack::drop_vars_except(const VarDescrList& var_info, int excl_var) {
   return dropped;
 }
 
-void Stack::show(int flags) {
+void Stack::show() {
   std::ostringstream os;
   for (auto i : s) {
     os << ' ';