[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-assign-1.tolk

@@ -0,0 +1,9 @@
+fun main() {
+    var c = 1;
+    (c, c) = (2, 3);
+}
+
+/**
+@compilation_should_fail
+@stderr one variable modified twice inside the same expression
+*/