str: add ASCII fast path to word_to_titlecase

[Rachit2323]

The word_to_titlecase function had a FIXME comment since a long time asking to add a fast path for ASCII text.

Before this change, even simple English text like "hello world" was going through slow Unicode lookup tables. That was unnecessary.

This fixes it by using the same trick that to_lowercase already uses — plain English letters are now handled much faster, and only Greek/Chinese/etc still use the Unicode tables.

No behavior changes, just faster for normal English text.

[rustbot]

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.