Configurable underlying container

Similary to #39 but rather use the same approach as Boost.Container does for its flat_* containers: simply add a template parameter for specifying the underlying container.
(I for example strictly _need_ to use a flat/trivially persistable underlying container).

https://lists.boost.org/Archives/boost//2021/09/251937.php

Start by reading issue #39 and the linked Boost.Container discussion to understand the proposed approach for flat_* containers. No source file, test, or entry point is named; the work is complete when the underlying container can be specified as a template parameter in the relevant ICL containers and the behavior is covered by appropriate tests.

Written by the indexing model from the issue text.