btree/node.rs: pub fn `dormant` should be marked as unsafe

[btj]

This issue is about the "public" API of module alloc/src/collections/btree/node.rs. (The module itself is not public.)

I don't know how serious people are about pursuing strong static typing for this API, but depending on that, I think both dormant functions (on NodeRef and on Handle) should be marked as unsafe: the very existence of a dormant NodeRef/Handle is unsafe because it can be turned into a NodeRef<Immut<'a>, ...> using non-unsafe method reborrow and then inspected arbitrarily, which may cause races, aliasing rules violations, etc. with concurrent use of a NodeRef<Mut<'a>, ...> existing at the same time.

[btj]

In fact, should a dormant NodeRef be a NodeRef at all? Are there any operations at all that a client should be able to perform on a dormant NodeRef, other than awakening it? Would it not make more sense to introduce an entirely separate struct DormantNodeRef instead? That way, the dormant functions could remain non-unsafe. (Same for Handle.)

A related issue, by the way, is that the discussion of the possible values for BorrowType in the doc comment for NodeRef currently does not mention DormantMut.