[idea] improve length of constraint name for safe_change_column_null

[Roguelazer]

The safe version of change_column_null automatically generates a constraint of the form "#{table}_#{column}_null"; for wordy table and column names, this can easily exceed the 64-character constraint name length limit enforced by PostgreSQL. Ideally, we could do one of the following:

1. Always use a short constraint name (for example, strong_migrations_#{SecureRandom.uuid.delete "-"}); ideally this constraint shouldn't ever actually be seen or interacted with by a human
2. OR allow the migration to specify a custom constraint name as a kwarg

Thoughts?

[ankane]

Hi @Roguelazer, thanks for reporting. I don't think this is common, but fixed in the commit above.