Draft: Add WrappedFS interface

[wrouesnel]

WrappedFS implements an optional interface to allow an afero.FS to be dereferenced to yield the underlying filesystem it is wrapping. It does not make sense in all circumstances, but can provide for example to allow determining if an afero filesystem is backed by an underlying OsFs.

Closes #553

Draft PR to highlight the basic idea and solicit feedback - does this make sense?

The specific use case I have is holding something like ReadOnlyFs(OsFS()) and wanting to make sure that the underlying path is actually a filesystem path (in which case, acknowledging it's unsafe, I should be able to hand it off to a subprocess).

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