- deprecated scalar=True argument on select(). its replaced

- deprecated scalar=True argument on select().  its replaced
by select().scalar() which returns a _ScalarSelect object, that obeys
the ColumnElement interface fully
- removed _selectable() method.  replaced with __selectable__() as an optional
duck-typer; subclassing Selectable (without any __selectable__()) is equivalent
- query._col_aggregate() was assuming bound metadata.  ick !
- probably should deprecate ClauseElement.scalar(), in favor of ClauseElement.execute().scalar()...
otherwise might need to rename select().scalar()