- Added support for "set-aggregate" functions of the form

- Added support for "set-aggregate" functions of the form
``<function> WITHIN GROUP (ORDER BY <criteria>)``, using the
method :class:`.FunctionElement.within_group`.  A series of common
set-aggregate functions with return types derived from the set have
been added. This includes functions like :class:`.percentile_cont`,
:class:`.dense_rank` and others.
fixes #1370
- make sure we use func.name for all _literal_as_binds in functions.py
so we get consistent naming behavior for parameters.