fix unfortunate bug in isFilePathSep

All filepaths were mulled into a single long string, bleh.
Luckily doesn't seem to affect overall perf that much: Now at @70ms
(from 68ms).

Still need to redo some perf and number stuff.
implement possible version of canonicalization for 8 ms: 70ms -> 78ms

This is with shared, lazy canonicalization. This allows hitting the
cache for 114820 of the 162758, so it'll likely fare better than other
approaches, even though it requires an additional string copy in the
non-cached path. (Still need to measure other approaches though.)