[ADD] base_field_serialized

This is meant to fix https://github.com/odoo/odoo/issues/1263 (or a part of it).

Note that fields.py is not necessary right now, but probably this code will be removed in some subsequent cleanup and then it's good to have it already in place.

So base_field_sparse should be very similar to this I guess, but I don't have time currently to do that myself.