add send/sync bounds to prevent data race to RC of ArcGuard<Rc, T>

[JOE1994]

Fixes #33

This PR adds appropriate Send/Sync trait bounds to the Send/Sync impls of ArcGuard<RC, T>,
in order to prevent unsound programs at compile time.

Thank you for reviewing this PR 👍

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[JOE1994]

@googlebot I signed it!

[darconeous]

Not sure why travis is having trouble.

[JOE1994]

Thank you for merging the fix 👍 Would you also consider publishing a new minor release on crates.io?