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.
