Add drain_filter method to HashMap and HashSet
Add `HashMap::drain_filter` and `HashSet::drain_filter`, implementing part of rust-lang/rfcs#2140. These new methods are unstable. The tracking issue is #59618.
The added iterators behave the same as `BTreeMap::drain_filter` and `BTreeSet::drain_filter`, except their iteration order is arbitrary. The unit tests are adapted from `alloc::collections::btree`.
This branch rewrites `HashSet` to be a wrapper around `hashbrown::HashSet` rather than `std::collections::HashMap`.
(Both are themselves wrappers around `hashbrown::HashMap`, so the in-memory representation is the same either way.) This lets `std` re-use more iterator code from `hashbrown`. Without this change, we would need to duplicate much more code to implement `HashSet::drain_filter`.
This branch also updates the `hashbrown` crate to version 0.9.0. Aside from changes related to the `DrainFilter` iterators, this version only changes features that are not used in libstd or rustc. And it updates `indexmap` to version 1.6.0, whose only change is compatibility with `hashbrown` 0.9.0.
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