[WIP] Consolidate sheath boundaries

[ZedThree]

Includes #368. ~e94eb10727065ea855cbdf388a8781b3bdfc0808 is the commit merging sheath_boundary into sheath_boundary_simple.~

~Current task is to see about merging sheath_boundary_insulating in as well.~

Merges sheath_boundary, sheath_boundary_simple, and sheath_boundary_insulating into a single implementation with compile-time switches between the different conditions.

Is SheathKind::normal a sensible name for the "non-simple" case? (Note that this is an internal name only, and not user-visible).

TODO:

• [ ] guard input options that aren't appropriate for the different sheath kinds