Fixed the structure of the reverse-engineered mapping

when using the DatabaseDriver, the field mapping being generated does not match the field mapping expected by all other drivers or the SchemaTool. This means that these mapping settings get ignored.

See https://github.com/doctrine/migrations/issues/184 for the initial report


@stof I added a [documentation chapter in DBAL](http://docs.doctrine-project.org/projects/doctrine-dbal/en/latest/reference/types.html#mapping-matrix) about type mappings some months ago. There you can see which platform supports it and which doesn't.
Looks like only Drizzle does not support distinction between fixed and variable string types.