Alter key support?

In some conditions keys are not modifiable while you could still modify them using plain sql.

Say you have foreign key. If you want to modify that key by adding another column to the key, you could do it like this way:

ALTER TABLE `records` 
  DROP INDEX `record_types`, 
  ADD UNIQUE `record_types` (`edition_id`, `date`);

However, phinx splits this into two queries and this surely breaks as this key is required and could not be removed.

The issue names no files or tests. Start by reproducing a foreign-key key modification using the combined MySQL ALTER TABLE statement and the two statements Phinx generates, then trace the migration path that alters keys. Done means the key can be modified without failing because the required key is removed first.