Implement -sort

[tavianator]

-s will sort by name, but it would be nice to sort by other attributes like type, inode number, etc. The main difficulty is that at the time we sort, we don't have the struct BFTW available, so we either need to potentially compute those earlier and cache them for the later actual visits, or pass a different (lighter) type to the comparison function. But even with a lighter type for comparisons, determining the type/inode number may need a stat(), which should be cached somehow anyway.

It would be extra nice to support compound orderings like -sort type,-name meaning sort by type first, then name backwards.