Auto merge of #121150 - Swatinem:debug-ascii-str, r=joboet

Add a fast-path to `Debug` ASCII `&str`

Instead of going through the `EscapeDebug` machinery, we can just skip over ASCII chars that don’t need any escaping.

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This is an alternative / a companion to rust-lang/rust#121138.

The other PR is adding the fast path deep within `EscapeDebug`, whereas this skips as early as possible.

Author: bors