Add cold attribute to less likely printer queue branches

[MichaReiser]

Summary

The formatter writes most elements to a single Vec<FormatElement> except for:

• Memoized content: It's sometimes necessary to emit the same content but using two different layouts. The formatter memoizes/interns the content's IR to avoid formatting it twice. The internet content stores the elements in its own Vec
• BestFitting layout: The BestFitting IR allows multiple different layouts for the same content. It stores the IR elements of the variants in a dedicated Vec

The fact that the Printer must support multiple Vecs add complexity to its queue because the queue needs to support a stack of slices where it pops elements from the top slice but takes the next slice if the top slice is empty.

This PR adds a few #[cold] attributes signaling that it's unlikely that the top slice is empty because a) the printer takes elements from the main top-level Vec most of the time and b) most slices contain many elements, but have only one end ;)

This gives us a solid 2-3% performance improvement in the printer (I no longer see FitsQueue::pop in the top methods when profiling the large dataset)

@BurntSushi won't believe this

Test

cargo test

[github-actions]

ruff-ecosystem results

Formatter (stable)

ℹ️ ecosystem check encountered format errors. (no format changes; 1 project error)

openai/openai-cookbook (error)

warning: Detected debug build without --no-cache.
error: Failed to read examples/How_to_handle_rate_limits.ipynb: Expected a Jupyter Notebook, which must be internally stored as JSON, but this file isn't valid JSON: trailing comma at line 47 column 4

Formatter (preview)

ℹ️ ecosystem check encountered format errors. (no format changes; 1 project error)

openai/openai-cookbook (error)

ruff format --preview

warning: Detected debug build without --no-cache.
error: Failed to read examples/How_to_handle_rate_limits.ipynb: Expected a Jupyter Notebook, which must be internally stored as JSON, but this file isn't valid JSON: trailing comma at line 47 column 4

[charliermarsh]

That's neat.

[BurntSushi]

Wow. Rather impressive. It's very rare to see #[cold] matter. 🎉 Nice find!