decimal: provide MP_DECIMAL extension serializer

Msgpuck functions mp_snprint() and mp_fprint() now support
customizable MP_EXT serializer. This patch adds such for
MP_DECIMAL. It is not activated yet, but will be later, when other
extensions also get their serializers.

Part of #4719

Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>
Reviewed-by: Serge Petrenko <sergepetrenko@tarantool.org>
Reviewed-by: Nikita Pettik <korablev@tarantool.org>
Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>