I'm not really very happy that `is_sep` is public to begin with to be honest. It's really just a helper method, but since `PosixPath` is based on `~[u8]` `posix::is_sep` needs to take `&u8`, and since `WindowsPath` is based on `Str` `windows::is_sep`/`windows::is_sep2` needs to take `char`. They're mostly just meant for the ability to pass to a split iterator of the appropriate variant.

I could have a `posix::is_sep_char`, but it feels weird making that just so `libextra::glob` can avoid doing extra work. Doesn't really feel like a win.