Expose column spec information for the _iter interface

[piodul]

Related to #271.

In #276, information about column names became available for the results of Session::query and Session::execute through the QueryResult::col_specs field.

We should also expose this information through the iterator object returned by Session::query_iter and Session::execute_iter methods.

[cvybhu]

I implemented it when adding query_all to Session, but eventually these changes were dropped.
There is a commit on this branch: https://github.com/cvybhu/scylla-rust-driver/commits/queryall-backup:

cvybhu@6bf7182

[piodul]

Fixed by b46f913