Make Volatile derive Copy.

[cedws]

Hi there. This PR makes the Volatile struct derive Copy. I'm not sure whether this will have any negative or unintended consequences.

To clarify what this is needed for, I am currently looking to improve some of the VGA text buffer code in Philipp Oppermann's blog_os. This PR allows an optimisation to be made for which I will send a PR there too.

[phil-opp]

Hi, thanks for the PR! The problem with implementing Copy for Volatile is that it makes accidental copies possible. Most of the time, a volatile refers to a specific memory location, so it does not make much sense to copy it e.g. to the stack. See #5 (comment) for an example.

I hope that rust-lang/rust#53645 is merged soon. It implements const generics, which means that we could create a VolatileArray<T, const N> type, which could solve the initialization problem easily through a VolatileArray::new(array: [T; N]) method.

[cedws]

Ah I see, I was trying to do something similar with [Volatile::new(_); N];. It seems VolatileArray would be more optimal. Thanks a lot, I have a few other changes for the kernel that you might like.