Improve type detection for Values / Tuple

Improve type detection for Values / Tuple

Fixed issue in new :class:`_sql.Values` construct where passing tuples of
objects would fall back to per-value type detection rather than making use
of the :class:`_schema.Column` objects passed directly to
:class:`_sql.Values` that tells SQLAlchemy what the expected type is. This
would lead to issues for objects such as enumerations and numpy strings
that are not actually necessary since the expected type is given.

note this changes NullType() to raise CompileError for
literal_processor; NullType() does not imply the actual value
NULL as much as it does "unknown type" so this should make failure
modes more clear.

Fixes: #5785
Change-Id: Ifbf5e78373102380b301098f30e15011efa98b5e