support named CHECK constraints in batch

support named CHECK constraints in batch

Named CHECK constraints are now supported by batch mode, and will
automatically be part of the recreated table assuming they are named. They
also can be explicitly dropped using ``op.drop_constraint()``. For
"unnamed" CHECK constraints, these are still skipped as they cannot be
distinguished from the CHECK constraints that are generated by the
``Boolean`` and ``Enum`` datatypes.

Note that this change may require adjustments to migrations that drop or
rename columns which feature an associated named check constraint, such
that an additional ``op.drop_constraint()`` directive should be added for
that named constraint as there will no longer be an associated column
for it; for the ``Boolean`` and ``Enum`` datatypes, an ``existing_type``
keyword may be passed to ``BatchOperations.drop_constraint`` as well.

Fixes: #884
Change-Id: I3694430f7c9735bcc3b0765893b333cc8e0d1cd3