[MERGE] orm,fields: fields.char's argument now all optional (especially @size) +...

The size limit on fields.char is entirely cosmetics,
as it makes no difference in performance or size for
PostgreSQL storage. The size only has to be specified
when the field needs to be size-constrained for
business reasons, like fixed-length references, EAN, etc.

bzr revid: odo@openerp.com-20120723172229-vyzrgzhugsd17rao