`/` is a valid sep on Windows as long as the path isn't verbatim (i.e. has the `\\?\` prefix), which it obviously can't be here.

I considered making `Path` implement `FromIterator<&str>` (and `~str`/`@str`, plus `&[u8]`/`~[u8]`/`@[u8]`), along with `Extendable`, but I wasn't convinced that the additional complexity was worthwhile.