[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-tester/tests/invalid-call-8.tolk

@@ -1,8 +1,10 @@
+fun get_incoming_value() { return 3; }
+
 fun main() {
     var incoming_ton: int = get_incoming_value().3();
 }
 
 /**
 @compilation_should_fail
-@stderr expected method name, got `3`
+@stderr type `int` is not indexable
  */