`Update` doesn't return the entity any more.

Instead, it just returns a void, to be homogenized with the `Delete`
action.
Added `updatev` and `deletev`.

These combinators are just like plain update and delete, respectively,
but they return the involved value: the new one in case of an update
and the old one in the case of a delete.
Merge pull request #38 from jeslg/homogenise

closes #36