Change `core::iter::Fuse`'s `Default` impl to do what its docs say it does

The [docs on `impl<I: Default> Default for core::iter::Fuse<I>`](https://doc.rust-lang.org/nightly/std/iter/struct.Fuse.html#impl-Default-for-Fuse%3CI%3E) say (as the `I: Default` bound implies) that `Fuse::<I>::default` "Creates a `Fuse` iterator from the default value of `I`". However, the implementation creates a `Fuse` with `Fuse { iter: Default::default() }`, and since the `iter` field is an `Option<I>`, this is actually `Fuse { iter: None }`, not `Fuse { iter: Some(I::default()) }`, so `Fuse::<I>::default()` always returns an empty iterator, even if `I::default()` would not be empty.

This PR changes `Fuse`'s `Default` implementation to match the documentation. This will be a behavior change for anyone currently using `Fuse::<I>::default()` where `I::default()` is not an empty iterator[^1], as `Fuse::<I>::default()` will now also not be an empty iterator.

(Alternately, the docs could be updated to reflect what the current implementation actually does, i.e. returns an always-exhausted iterator that never yields any items (even if `I::default()` would have yielded items). With this option, the `I: Default` bound could also be removed to reflect that no `I` is ever created.)

[Current behavior example](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=a1e0adc4badca3dc11bfb70a99213249) (maybe an example like this should be added to the docs either way?)

This PR changes publicly observable behavior, so I think requires at least a T-libs-api FCP?

r? libs-api

cc rust-lang#140961

`impl<I: Default> Default for Fuse<I>` was added in 1.70.0 (rust-lang#99929), and it's docs and behavior do not appear to have changed since (`Fuse`'s `iter` field has been an `Option` since before the impl was added).

[^1]: IIUC it is a "de facto" guideline for the stdlib that an iterator type's `default()` should be empty (and for iterators where that would not make sense, they should not implement `Default`): cc rust-lang/libs-team#77 (comment) , so for stdlib iterators, I don't think this would change anything. However, if a user has a custom `Iterator` type `I`, *and* they are using `Fuse<I>`, *and* they call `Fuse::<I>::default()`, this may change the behavior of their code.