MirageOS signatures for filesystem devices

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mirage-fs provides the `[Mirage_fs.S][fs]` and `[Mirage_fs_lwt.S]` signatures
the MirageOS filesystem devices should implement.

[fs]: http://mirage.github.io/mirage-fs/Mirage_fs.html
[fslwt]: http://mirage.github.io/mirage-fs/Mirage_fs_lwt.html