This updates the MySQL, Postgres, and SQLite database backends to

support changing the types of a field.
Both MySQL and Postgres build upon the common code, building SQL for

altering the field type, and letting the common code handle constraint

management. This is more complicated in the case of Postgres, where we

also need to backup/restore the AUTO INCREMENT state, and need to

cast field types. MySQL does not appear to require either of these

operations.
The SQLite support, as per usual, requires a full table rebuild in order

to change columns. This completely overrides the common code, going

through our standard table rebuild logic. There isn't much that's needed

here, since we already have machinery set up for determining how to

replace an existing field, and the type isn't really special in this

regard. The only addition to the logic is a new CHANGE COLUMN TYPE

operator that's passed in order to set the right field state.
Upcoming changes will introduce ChangeField support for invoking this

behavior, along with unit tests.

Unit tests pass when used with the other in-progress changes.