Windows file system compatibility (for HVFS)

I've replaced all forward slashes in file paths with 'pathSeparator', 'isPathSeparator', and '</>' from `System.FilePath`. This allows using HVFS and HVFSChroot on Windows (and by extension serving directories with hsftp).

I have used the functions from `filePath` where it seemed appropriate, but not where MissingH behaves notably (by tests) differently: e.g. `splice_path`/`unsplice_path` versus `splitPath`/`joinPath`.

I replaced `++ "/" ++` with `</>`. This isn't exactly equivalent as `</>` will handle drop the first argument if the second is an absolute path. This doesn't break any of the tests, but if you think it will be a problem `++ [pathSeparator] ++` can be used instead.

Since the change precludes pattern matching on `'/'` I had to restructure some of the functions to use `if … then …else` instead of `case … of `.

I have not made these replacements in System.Daemon, which I'm not sure is relevant to Windows (uses '/dev/null').

The tests have been updated so they pass on Windows (with the exception of  `4:Time:0:ctu2e:0` and `4:Time:0:ctu2e:0` which are already broken and unaffected by this change).