GridAab & Vol split

[kpreid]

GridAab, the type of axis-aligned boxes with i32 coordinates, has a volume limit: no GridAab may have a total volume (or number of enclosed cubes) greater than usize::MAX, so that every GridAab may correspond to a linearization into a vector.

This has proven more inconvenient than helpful, because all operations that might construct a larger GridAab have to be fallible, but the volume limit is irrelevant to most of those operations. Therefore, I am splitting it into two types:

• GridAab will become an arbitrary axis-aligned box. It will no longer have a volume limit, and also will not have any operations related to linearization (other than, maybe, iteration).
• Vol, which is primarily a container for volume data, can also have no data but still only exist when the volume limit is satisfied, thus serving as a “volume-checked GridAab”.

History and remaining tasks:

• Create Vol — commit 1128553
• Remove GridAab::index() — commit d3ba1e7
• Make GridAab::volume() fallible — commit ac68ecc
• Check that no remaining users of GridAab — especially .interior_iter() — care about volume.
• Remove GridAab volume limit — commit 62bc45e
• Adjust signatures of GridAab methods to be infallible where that is now possible — commit e373374

[kpreid]

All done.