Specialize advance_back_by like the forward version

[cuviper]

We specialized advance_by in #141086, so it makes sense to have
advance_back_by use the same trick in reverse, i.e. try_rfold.

This also updates nth_back to mirror nth, and the rest of the
DoubleEndedIterator methods already match Iterator.

[rustbot]

r? @Darksonn

rustbot has assigned @Darksonn.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: libs
• libs expanded to 12 candidates
• Random selection from 6 candidates

[rustbot]

⚠️ Warning ⚠️

• There are issue links (such as #123) in the commit messages of the following commits.
  Please move them to the PR description, to avoid spamming the issues with references to the commit, and so this bot can automatically canonicalize them to avoid issues with subtree.
  • cbc8109

[cuviper]

The previous PR used a Cycle<slice::Iter> to demonstrate the benefit, but that doesn't work in reverse. Here's a different example with Flatten: https://iter.godbolt.org/z/ojWT3n9xz