Provide conversions between PosixPath and WindowsPath

https://github.com/haskell/tar/pull/88 introduces
```haskell
-- | We assume UTF-8 on posix and UTF-16 on windows.
toWindowsPath :: MonadThrow m => PosixPath -> m WindowsPath
toWindowsPath posix = do
str <- PFP.decodeUtf posix
win <- WFP.encodeUtf str
pure $ WS.map (\c -> if WFP.isPathSeparator c then WFP.pathSeparator else c) win

-- | We assume UTF-8 on posix and UTF-16 on windows.
toPosixPath :: MonadThrow m => WindowsPath -> m PosixPath
toPosixPath win = do
str <- WFP.decodeUtf win
posix <- PFP.encodeUtf str
pure $ PS.map (\c -> if PFP.isPathSeparator c then PFP.pathSeparator else c) posix
```

IMHO such utilities should better be provided by `filepath` itself, ideally optimized to a single pass without any intermediate structures.