Fix MySQL native ENUM autogenerate detection

Fix MySQL native ENUM autogenerate detection

Implemented type comparison for :class:`.ENUM` datatypes on MySQL, which
checks that the individual enum values are equivalent.  If additional
entries are on either side, this generates a diff.  Changes of order do not
generate a diff.  Pull request courtesy Furkan Köykıran.

Fixes: #779
Fixes: #1745
Closes: #1746
Pull-request: https://github.com/sqlalchemy/alembic/pull/1746
Pull-request-sha: 49b17845d89e5938c1616bb77fbb6294d8c4db2f

Change-Id: I0023b659acec603d39feaea45413637fe559ea56