When turning UTF-8 paths into UCS-2 paths for Windows, always use
the \\?\-prefixed paths. Because this bypasses the system's
path canonicalization, handle the canonicalization functions ourselves.
We must:
1. always use a backslash as a directory separator
2. only use a single backslash between directories
3. not rely on the system to translate "." and ".." in paths
4. remove trailing backslashes, except at the drive root (C:\)
