Migration fails if entities are not topologically sorted

[meteficha]

If you have

Foo
  ref BarId

Bar

then persistent's migration fails because it first tries to create Foo and then Bar.

For this bug it suffices to do a topological sort inside the migration code.

[gregwebs]

For workarounds, you can use the printMigration function to get the migration Yesod was going to produce and then manually change it.

[gregwebs]

@snoyberg Was this addressed in the recent refactoring?

[snoyberg]

No, it has not been solved. I'm hesitant to implement it without solving circular dependencies (#24) as well, though that shouldn't really be necessary.