integer-range-set.hpp: add `begin()` and `end()` to bt2::CommonIntegerRangeSet

integer-range-set.hpp: add `begin()` and `end()` to bt2::CommonIntegerRangeSet

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I85d92d27f90f25b2f86780180b248bde3c394b20
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7568
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10795
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>