I spent a long time working on this data structure, and I still think
it's a neat idea, but it has no business being in the std lib.
I'm aware of the few remaining references to SegmentedList that exist in
the std lib, but they are dead code, and so I'm leaving the dead
references as a clue that the code is dead. Cleaning up dead code will
be a separate effort that involves code coverage tools to make sure we
find it all.
std-lib-orphanage commit: 2c36a7894c689ecbaf63d5f489bb0c68773410c4
closes#7190
50a336fff8