Add `Frame.to_tuples()` method

This method should convert a Frame into a list of tuples, where each tuple will represent a single row. For example:
```
>>> fr = dt.Frame(A=[1, 2, 3], B=["foo", "bar", "buzz"])
>>> fr.to_tuples()
[(1, "foo"), (2, "bar"), (3, "buzz")]
```
Thus, it is equivalent to `list(zip(*fr.to_list()))`.