multiboot2: foundation for mutable reference to tags

[phip1611]

This is the foundation for a mutable boot information and another approach than #140. By using the NonNull facility, we can ensure that the compiler will not optimize away any mutable operations. I could not find a way to use UnsafeCell here in a meaningful way.

I think this approach has the advantage that the public API is barely changed - unlike in #140.

[phip1611]

I'm still unsure but I think with this approach, we might have a less invasive approach while still having idiomatic Rust - What do you think, @YtvwlD ?

[YtvwlD]

oh, yes, this looks interesting (if it worked :upside_down_face:)

[phip1611]

Obsolete as new API was too breaking. This needs a new approach. However, I think it can be nicely integrated in the new multiboot2-common lib