support slice access for .c

support slice access for .c

Added support for slice access with :class:`.ColumnCollection`, e.g.
``table.c[0:5]``, ``subquery.c[:-1]`` etc. Slice access returns a sub
:class:`.ColumnCollection` in the same way as passing a tuple of keys. This
is a natural continuation of the key-tuple access added for :ticket:`8285`,
which it appears to be an oversight that this usage was omitted.

Change-Id: I6378642f39501ffbbae4acadf1dc38a43c39d722
References: #8285
References: #9690