Only the `Sys()` value from os.FileInfo is kept as field `sys` to
support Windows. The os.FileInfo removal ensures that for values like
`ModTime` that existed in both data structures there's no more confusion
which value is actually used.
This adds two implementations of the new `FS` interface: One for the local
file system (`Local`) and one for a single file read from an
`io.Reader` (`Reader`).
