htools: abstract and simplify a multigroup function

In the future, this 'split/associated instance idx list' functionality
will be used by other code, so let's move it to its own code.

The new method of computing the split also requires exporting one more
function from IntMap in Container.hs.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>