Expose the underlying file object.

[de-vri-es]

This PR adds the ability to use an already opened File object to create a new Spidev. Additionally, it adds read-only inner() and consuming into_inner() to access the underlying File object.

My primary reason for wanting this is that it allows to call flock() before using the spidev. That's something I want to do to avoid multiple processes controlling the same device.

And of course, if you can wrap an open File, it also makes sense to be able to access and retrieve it again.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nastevens (or someone else) soon.

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[posborne]

Thanks @de-vri-es, looks reasonable. bors r+

[posborne]

Ah, haven't setup bors on this one yet.

[de-vri-es]

Thanks for the quick review and merge :)