`MpMcQueue`: add `MpMcQueueView`, similar to `VecView` on top of #486

[sosthene-nitrokey]

This would supersede #484. We need to pay attention to the Drop impl depending on the order we merge PRs

This adds a couple of methods to the Storage sealed trait to deal with the fact that Borrow cannot be used in concurrent code:

• One to safely get the length of the buffer from a pointer to it
• One to get a pointer to the beginning of the slice