stable copy copy_from_slice and copy_to_slice

[TheButlah]

I was considering using this crate, but some of the features I was interested in are gated behind the unstable feature flag:

• copy_from_slice
• copy_into_slice

read_volatile and write_volatile have been stabilized for a long time. Perhaps the implementation could only use the unstable intrinsics when the unstable feature flag is detected, and use the stable read/write_volatile apis if the flag is not provided? That way its more of an "unstable optimization" than a "this api only exists on nightly compiler"