Maintain bitvector of original slashes from manifest when canonicalizing

Intended as a more satisfactory resolution to #843.

(Still testing, but useful enough to take a look now.)
I think the idea here was that things are canonicalized before they're stored in the depslog, so that they don't have to be canonicalized on load. 

Is it possible to not require this, e.g. by always using slash_bits that are computed from manifest loading?