Deprecate `DList::ListInsertion`
auto merge of #20185 : csouth3/rust/dlist-deprecate, r=alexcrichton

This PR deprecates the `DList::ListInsertion` trait, in accordance with rust-lang/rfcs#509.  The functions which were previously part of the ListInsertion impl for `DList::IterMut` have been moved to be inherent methods on the iterator itself, and appropriate doctests have been added.