Added IntervalSet, sets of non-adjacent intervals (#121)

* Added `IntervalSet`, sets of non-adjacent intervals

Relocated from nimbus-eth1 snap sync development

* Fix local import directive

* Fix --styleCheck complaints

* Attempt to get around CI problem by varying items

details:
  Vary all_tests exec list
  Hide useless globalness of `noisy` constant in non-debugging mode

Author: mjfh