Include support for `hashbrown` crate

[OliverNChalk]

Looking for a pointer to where std::collections::HashMap support is implemented and how one might extend this to support hashbrown's more performant implementation?

[jonasbb]

You can look at #627 to see the changes needed for supporting indexmap v2. The changes for hashbrown should be analogous.

1. You add hashbrown as an optional dependency using the feature name hashbrown_0_14. Document the feature flag under serde_with/src/guide/feature_flags.md and in serde_with/CHANGELOG.md.
2. Extend the foreach_map* and foreach_set macros in serde_with/src/de/impls.rs and serde_with/src/ser/impls.rs to include HashSet and HashMap types from hashbrown.
3. Add the needed trait implementations under serde_with/src/duplicate_key_impls/
4. Add a couple of tests.

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I was under the impression that std::collections::HashMap and hashbrown::HashMap have identical performance unless you need access to the raw table functionality of hashbrown. Is that no longer the case?

[OliverNChalk]

Ahh, my bad you are right about about the implementations being identical. I do make use of the extra API surface in some cases to optimize some things but I must have been thinking about the different hasher used not the actual map impl.

I still prefer the hashbrown crate over std due to:

• Additional API features
• Preferable default hasher

I'm happy to open a PR if you still are open to it (given the premise of improved performance is somewhat dubious).

[jonasbb]

A PR is still welcome :) serde_with already supports a bunch of extra crates via optional features.

[OliverNChalk]

Opened #637