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Walk every entry directly in MultiDict_ForEach (#1544)

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## What do these changes do?

`_md_foreach_all()` drove `md_next()`, the cursor `iter.h` uses for
`__next__`. There each step re-enters the critical section, so the
cursor has
to re-check the version, re-read `md->keys` and recompute the entry
array on
every call, and it writes three out-parameters of which this caller
wants
two. A caller that holds the section for the whole walk needs none of
that.

`md_walk_all()` puts the linear scan next to `md_walk()`, sharing its
visitor
contract and its post-visitor staleness check. That check is also
stricter
than what the old loop had: it compares `md->keys` as well as the
version.

The reentrancy guard the walk promises had no test, so both harnesses
gained
a visitor that adds to the multidict being walked, covering it for the
whole-table walk and for the single-key one.

The other leg, `_md_foreach_key()`, is unchanged: `md_walk()` is seeded
from
a hash, filters on `entry->hash` and `_str_cmp`, and carries a seen set
that
exists only because a probe chain can revisit a slot. A linear scan
needs
none of those, so the two traversals stay separate.

## Are there changes in behavior for the user?

No.

## Is it a substantial burden for the maintainers to support this?

No. `_md_foreach_all()` drops from 24 lines to 5, and both legs of
`MultiDict_ForEach()` now go through `walk.h`.

## Related issue number

N/A

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist
- [ ] Documentation reflects the changes — N/A, no user-visible API
change
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt` — N/A, no such file in this repo
- [x] Add a new news fragment into the `CHANGES/` folder
- [x] `make doc-spelling` passes and any new technical words are added
to `docs/spelling_wordlist.txt`

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