aio-libs / multidict / 37472940115

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Split the sequence update into prepare, next item and per-op loops (#1706)

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## What do these changes do?

Replaces the forced-inline `_md_update_from_seq()` body, which was
stamped out four times (extend for each class, update, merge), with
three explicit steps. `_md_seq_prepare()` picks the list, tuple or
iterator path once and is out of line. `_md_seq_next()` fetches and
parses item `i` and computes its identity and hash, also out of line,
with no per-class copies. Each of extend, update and merge then runs its
own `for (i = 0;; ++i)` loop over them with the operation written in
place. Extend no longer has a copy per class, and neither the loops nor
`md_update_from_seq()` force inlining either way any more; GCC currently
inlines the update and merge loops into their callers and keeps extend
out of line. The other `NOINLINE` attributes in `bulk_update.h` (the
dict, multidict and keyword-name wrappers and the cold error helpers)
are dropped too, leaving every inlining choice in the file to the
compiler apart from the existing `ALWAYS_INLINE` bodies.

The input pair is still released after the operation, so finalizers run
in the same order as in the pure-Python implementation. C-extension
only; the pure-Python backend has no counterpart to change.

## Are there changes in behavior for the user?

No.

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

No; the per-operation code is now plain loops instead of a switch inside
one shared body.

## Related issue number

N/A

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist (existing coverage; no behaviour
change)
- [x] Documentation reflects the changes (N/A, internal)
- [ ] 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... (continued)

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