Always escape grapheme extenders in str::escape_debug

[Jules-Bertholet]

This matches the behavior of impl Debug for str.

@rustbot label A-Unicode T-libs-api needs-fcp

[rustbot]

r? @Mark-Simulacrum

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The reviewer was selected based on:

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[Mark-Simulacrum]

I'll go ahead and kick off an FCP here, and cc @Manishearth (but I imagine your opinion here will match that given on the other PRs).

@rfcbot fcp merge libs-api

[rust-rfcbot]

@Mark-Simulacrum has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @the8472

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