:file -> :test-file
Rename string-to-cltype -> string-to-dbtype. It returns a keyword (or a list which has a keyword at its car).
Add tests for migration for each databases.
Ignore :auto-increment t if the connection driver is SQLite3.
Bind sxql:*quote-character* if a connection established.
Ignore dropped columns of PostgreSQL.
PostgreSQL doesn't support MODIFY COLUMN.