iteration over tuple components

Asteroid should support the following,
```
ast> load system io.
ast> for i in (1,2,3) do
....    io @println i.
.... end
error: only iteration over strings and lists is supported
ast>
```
Python supports this,
```
>>> for i in (1,2,3):
...     print(i)
... 
1
2
3
>>>
```
