Add lifetimes for GDT

[YtvwlD]

I was happy to be able to use this crate for creating a GDT like this:

let gdt = DescriptorTablePointer::new_from_slice(
    &[Descriptor::NULL, code_segment, data_segment]
);
unsafe {
    x86::dtables::lgdt(&gdt)
    // inline assembly with a far-jump goes here
}

This worked in a debug build, but failed in a release build because the array containing the descriptors did not live long enough. I spent some time finding this issue, so I figured it might be nice to express these constraints via lifetimes.

I'm not sure about disallowing access to the limit and base pointer but they probably shouldn't be writable.

[gz]

Thanks, any reason why you'd need a non-static lifetime in the new_from_slice? e.g., would this be simpler:

pub fn new_from_slice(slice: &'static [T])

[YtvwlD]

I'm not exactly sure. If you're writing a kernel, then you'll most likely load it with lgdt, so it would have to be 'static either way.

Not requiring it might be helpful for tests or VMs or something like that, but I don't know. I would be fine with the static lifetime just in new and new_from_slice.