Add wrappers for the Set data-type

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Implement getters for Counter and Set data-types

The getters wrap the supplied callback, and if the get succeeds, they
call the supplied function with the value already extracted.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>