Table operations to support iterators

Table operations currently expects sequences as inputs, but now that we are python3, it would be nice to also support iterators. I'm not sure how feasible this is, there was some discussion already in
https://github.com/astropy/astropy/pull/3369

Start by reading the discussion in pull request #3369, then locate the table operations that currently require sequence inputs. Determine which operations should accept iterators and define the expected behavior and tests; done means the agreed operations work with iterator inputs without regressing sequence support.

Written by the indexing model from the issue text.