implement tuple-slices from .c collections

implement tuple-slices from .c collections

Added new syntax to the ``.c`` collection on all :class:`.FromClause`
objects allowing tuples of keys to be passed to ``__getitem__()``, along
with support for ``select()`` handling of ``.c`` collections directly,
allowing the syntax ``select(table.c['a', 'b', 'c'])`` to be possible. The
sub-collection returned is itself a :class:`.ColumnCollection` which is
also directly consumable by :func:`_sql.select` and similar now.

Fixes: #8285
Change-Id: I2236662c477ffc50af079310589e213323c960d1