- add a type_coerce() step within Enum, Boolean to the CHECK constraint,
so that the custom type isn't exposed to an operation that is against the
"impl" type's constraint, [ticket:2842]
- this change showed up as some recursion overflow in pickling with labels,
add a __reduce__() there....pickling of expressions is less and less something
that's very viable...

Conflicts:
	lib/sqlalchemy/sql/elements.py
	lib/sqlalchemy/sql/sqltypes.py