Returning Table<'i, (Table<'j, Tup>, Table<'k, Tup>)>

[workingjubilee]

@eeeebbbbrrrr described a plan for how to make this happen by flattening out the nesting iterators into a single table, effectively an OUTER JOIN. This sounds desirable but we need more core work to happen so that we are well-positioned to do this.

[workingjubilee]

Also we should consider a shorter name for TableIterator (like perhaps just Table, or TableIter) so that typing out a signature like that doesn't drive someone bonkers.

[eeeebbbbrrrr]

TableIter

[eeeebbbbrrrr]

We want the equivalent of:

# select generate_series(1, 3), generate_series(100, 101);
 generate_series | generate_series 
-----------------+-----------------
               1 |             100
               2 |             101
               3 |                
(3 rows)

But with as many columns (and nestings) as the user decides.

[workingjubilee]

Current status: This idea has been made possible at all by #1701 but it's not clear when we'll be able to do more for it until after #1661