Fix skipiter not applicable in autoderef

[A4-Tacks]

Example

std example:

fn foo(nums: std::rc::Rc<[i32]>) {
    nums.$0
}

---

minicore example:

struct Foo;
impl Foo { fn iter(&self) -> Iter { Iter } }
impl IntoIterator for &Foo {
    type Item = ();
    type IntoIter = Iter;
    fn into_iter(self) -> Self::IntoIter { Iter }
}
struct Ref;
impl core::ops::Deref for Ref {
    type Target = Foo;
    fn deref(&self) -> &Self::Target { &Foo }
}
struct Iter;
impl Iterator for Iter {
    type Item = ();
    fn next(&mut self) -> Option<Self::Item> { None }
}
fn foo() {
    Ref.$0
}

Before this PR

me deref() (use core::ops::Deref)                 fn(&self) -> &<Self as Deref>::Target
me into_iter() (as IntoIterator)           fn(self) -> <Self as IntoIterator>::IntoIter
me iter()                                                             fn(&self) -> Iter

After this PR

me deref() (use core::ops::Deref)                 fn(&self) -> &<Self as Deref>::Target
me into_iter() (as IntoIterator)           fn(self) -> <Self as IntoIterator>::IntoIter
me iter()                                                             fn(&self) -> Iter
me iter().by_ref() (as Iterator)                             fn(&mut self) -> &mut Self
me iter().into_iter() (as IntoIterator)    fn(self) -> <Self as IntoIterator>::IntoIter
me iter().next() (as Iterator)        fn(&mut self) -> Option<<Self as Iterator>::Item>
me iter().nth(…) (as Iterator) fn(&mut self, usize) -> Option<<Self as Iterator>::Item>

[Veykril]

Won't this break cases where we have a type T that derefs to U where T has interesting impls but U does not?

I think the proper fix here is to walk the autoderef chain and find the first candidate that impls IntoIterator instead

[A4-Tacks]

I think the proper fix here is to walk the autoderef chain and find the first candidate that impls IntoIterator instead

Can't .find_map(|ty| ty.into_iterator_iter(ctx.db)) do it?