aio-libs / multidict / 38274499752

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Grow and move the table to KIND_ANYSTR in one step before an add (#1785)

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## What do these changes do?

An add used to check for room first, then `_md_store_new_entry()`
checked again whether the key fits a compact table and called
`md_to_anystr()` if not. Both checks are now one test on the add path,
and the rare work goes to a `COLD` `_md_prepare_add()`, which grows and
moves the table in a single `_md_rebuild_to_anystr()` when both are due.
`_md_store_new_entry()` no longer branches on the fit and cannot fail.

A side effect: the first plain `str` add to an empty `CIMultiDict` now
builds one `KIND_ANYSTR` table instead of a compact one that was copied
straight away (`empty_htkeys` is `KIND_ANYSTR`, so the old kind test
missed it).

`COLD` is needed: without it GCC inlined the slow path into
`md_add_with_hash_steal_refs()`, which then fell out of
`md_add_with_hash()` and `_md_extend_item()`, costing GIL `ctor_items`
+11% and `setitem_insert` +4%.

C extension only; the pure-Python backend has no table kinds.

## Are there changes in behavior for the user?

No.

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

No; it removes a branch from the store helper and reuses an existing
rebuild.

## Related issue number

N/A

## Checklist

- [x] I think the code is well written
- [ ] Unit tests for the changes exist: N/A, internal refactor covered
by the existing suite
- [ ] Documentation reflects the changes: N/A
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt`: N/A
- [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`

Callgrind, 3.14.7, Ir per op, base `276c2cea` against this branch; rows
that moved by 0.5% or more:

| operation | GIL | FT |
|---|---|---|
| ctor / reinit, MultiDict | -1.1..-1.2% | -13.0..-14.4% |
| ctor / reinit, CIMultiDict | +1.0..+1.4% | -... (continued)

842 of 1684 branches covered (50.0%)

Branch coverage included in aggregate %.

9799 of 10709 relevant lines covered (91.5%)

1.83 hits per line