- decoupled all ColumnElements from also being Selectables. this means

- decoupled all ColumnElements from also being Selectables.  this means
that anything which is a column expression does not have a "c" or a
"columns" attribute.  Also works for select().as_scalar(); _ScalarSelect
is a columnelement, so you can't say select().as_scalar().c.foo, which is
a pretty confusing mistake to make.  in the case of _ScalarSelect made
an explicit raise if you try to access 'c'.