[IMP] models: use slots for BaseModel

This restricts the attributes of a BaseModel instance to `env`, `_ids`
and `_prefetch_ids`.  This way, one can only assign fields on a record;
other assignments are programming errors.

This also reduces the memory footprint of records from 168 to 64 bytes
(-62%), and makes their instanciation faster.

closes odoo/odoo#51075

Related: odoo/enterprise#10529
Signed-off-by: Raphael Collet (rco) <rco@openerp.com>