Since this appears twice would it be better & safer to make `path::posix::is_sep` take `char` with an additional `is_sep_byte` (or keep `is_sep` as is and have a `is_sep_char`)?

Once that happens, you could use `use is_sep = path::windows::is_sep2` and `use is_sep = path::posix::is_sep_char` with the appropriate `cfg` on each, rather than defining wrappers?