Auto merge of #128711 - clarfonthey:default-iters-hash, r=dtolnay

impl `Default` for `HashMap`/`HashSet` iterators that don't already have it

This is a follow-up to #128261 that isn't included in that PR because it depends on:

* [x] rust-lang/hashbrown#542 (`Default`)
* [x] `hashbrown` release containing above

It also wasn't included in #128261 initially and should have its own FCP, since these are also insta-stable.

Changes added:

* `Default for hash_map::{Iter, IterMut, IntoIter, IntoKeys, IntoValues, Keys, Values, ValuesMut}`
* `Default for hash_set::{Iter, IntoIter}`

Changes that were added before FCP, but are being deferred to later:

* `Clone for hash_map::{IntoIter, IntoKeys, IntoValues} where K: Clone, V: Clone`
* `Clone for hash_set::IntoIter where K: Clone`

Author: bors