Stabilise os_string_pathbuf_leak

[its-the-shrimp]

This PR stabilises #[feature(os_string_pathbuf_leak)], which defines 2 new methods in the std:

impl OsString {
    pub fn leak<'a>(self) -> &'a mut OsStr;
}

impl PathBuf {
    pub fn leak<'a>(self) -> &'a mut Path;
}

ACP: rust-lang/libs-team#389
Tracking issue: #125965
Implementation: #125966

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[cuviper]

This still needs FCP...

@rustbot label -T-libs +T-libs-api
r? libs-api

[dtolnay]

@rfcbot fcp merge

[rfcbot]

Team member @dtolnay has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

No concerns currently listed.

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