Slight speed-up for lowercase and uppercase identifier checks

[charliermarsh]

It turns out that for ASCII identifiers, this is nearly 2x faster:

Parser/before     time:   [15.388 ns 15.395 ns 15.406 ns]
Parser/after      time:   [8.3786 ns 8.5821 ns 8.7715 ns]

[github-actions]

ruff-ecosystem results

Linter (stable)

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

sphinx-doc/sphinx (error)

ruff failed
  Cause: Selection of unstable rules without the `--preview` flag is not allowed. Enable preview or remove selection of:
	- FURB113
	- FURB131
	- FURB132

Linter (preview)

✅ ecosystem check detected no linter changes.

[MichaReiser]

Now you're micro micro optimising. I'm not sure if this is worth the extra complexity

[charliermarsh]

The counterargument is that we run this on every single identifier declared within every single function.

[BurntSushi]

LGTM. I don't mind the extra complexity so much in routines like this personally, because it's isolated and doesn't spread.

[charliermarsh]

Need to add some non-ASCII tests (which we should have anyway).