- some expression fixup:

- some expression fixup:
- the '.c.' attribute on a selectable now gets an
entry for every column expression in its columns
clause; previously, "unnamed" columns like functions
and CASE statements weren't getting put there.  Now
they will, using their full string representation
if no 'name' is available.
- The anonymous 'label' generated for otherwise
unlabeled functions and expressions now propagates
outwards at compile time for expressions like
select([select([func.foo()])])
- a CompositeSelect, i.e. any union(), union_all(),
intersect(), etc. now asserts that each selectable
contains the same number of columns.  This conforms
to the corresponding SQL requirement.
- building on the above ideas, CompositeSelects
now build up their ".c." collection based on
the names present in the first selectable only;
corresponding_column() now works fully for all
embedded selectables.