Regression in `BTreeMap` iterator variance

[apasel422]

#30426 caused btree_map::{IntoIter, Iter, Range} to become invariant in K and V. They were previously covariant.

CC #30642 @gankro @gereeter

[Gankra]

Oops! I could've sworn we checked up on variance, but I guess these slipped through.

[gereeter]

I checked for variance on BTreeMap itself, but forgot about the iterators.

[gereeter]

This is actually annoyingly tricky, because the invariance comes from NodeRef, which is generic in mutability - NodeRef<Mut, K, V, Type> should be invariant in K and V, but I don't think that it is possibly to have the variance of one type parameter depend on another.

Probably the best option is to make NodeRef universally covariant, just adding invariance markers to RangeMut and IterMut.