perf: Improve pest AST parsing

[aborgna-q]

#3212 tried to bump the pest version to 2.9.0 but found out that a soundness fix in it produced a 15-25% slowdown on sexpr decoding.

This PR pins the pest version to =2.8.8, so 2.9.0 doesn't get used for now.

While looking into that problem, I found out that a slight semantic-preserving change in the grammar may speed up things, so it's included here.

[codspeed-hq]

Merging this PR will improve performance by ×3.5

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 4 improved benchmarks
✅ 39 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	serialization/big_hugr/1048576/sexpr/without_extensions/decode	256.7 ms	28.6 ms	×9
⚡	serialization/big_hugr/1048576/sexpr/with_extensions/decode	256.8 ms	28.7 ms	×8.9
⚡	serialization/big_hugr/1024/sexpr/without_extensions/decode	738.5 µs	513.6 µs	+43.79%
⚡	serialization/big_hugr/1024/sexpr/with_extensions/decode	845.9 µs	619.4 µs	+36.55%

Tip

Curious why performance improved? Comment @codspeedbot explain why performance improved on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing ab/bump-pest (5aaa71a) with main (cb9a6a1)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.63%. Comparing base (6230f00) to head (5aaa71a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3235   +/-   ##
=======================================
  Coverage   81.63%   81.63%           
=======================================
  Files         242      242           
  Lines       47659    47659           
  Branches    41252    41252           
=======================================
  Hits        38906    38906           
  Misses       6729     6729           
  Partials     2024     2024           

Flag	Coverage Δ
python	89.38% <ø> (ø)
rust	80.43% <ø> (ø)

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[maximilianruesch]

Approved assuming our own CI performance warning goes away.

Is there some issue from pest you can link here? (maybe via invisible link so that it does not show up in the pest repo?)

[aborgna-q]

The relevant pest fix is at pest#1184.
That change caused the big performance drop in our parser.

Now that the benchmarks ran with the pinned 2.8.8 version, the slowdowns are gone.