Allow for testing migration down functions

[moore3071]

It's currently very easy to run into a poorly written migration where the down function doesn't correctly undo the migration. Diesel's documentation suggests the use of redo for testing this, but unless a project is testing this via CI, it's likely that faulty down functions will only be noticed when they're run. Even worse, it may be difficult to track down which migration was faulty. It would be cool if Diesel could include a command that starts with two blank databases, runs through the migrations step by step while redoing the migrations on the first database and checking that it is equivalent to the second database. A naive bit of pseudocode:

let db1 = get("database1")
let db2 = get("database2")
for x in migrations:
        db1.run_migration(x)
        db2.run_migration(x)
        db1.redo(x)
        assert_eq(db1,db2) # e.g. for Postgres, diff the pg_dump of the two

• I have already looked over the issue tracker for similar issues.

[moore3071]

Instead of using two databases and dumping database info, it would make a lot more sense to use Diesel's ability to print the schema. This only gives table structure, but would be much easier to implement.

An obviously needed feature in a binary implementing this would be the ability to exclude the testing of specified migrations.