Adapt the dumper used to the value of the objects

Adapt the dumper used to the value of the objects

Added a second dispatch to allow a dumper to upgrade to a specialised
version. Currently used to dump int to the smallest Postgres type
holding that value and to dump lists of object into typed arrays.

This change allows to write queries more naturally as no ``::int`` cast
should be needed anymore e.g. in date + int or jsonb ->> int.

Only Python implementation; C version to be implemented yet.