Stabilize feature sort_unstable

[ghost]

Closes #40585

[rust-highfive]

r? @alexcrichton

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[Mark-Simulacrum]

Looks like Travis failed.

[00:03:24] tidy error: /checkout/src/liballoc/slice.rs:1273: different stability level than before
[00:03:24] tidy error: /checkout/src/liballoc/slice.rs:1273: different `since` than before
[00:03:24] tidy error: /checkout/src/liballoc/slice.rs:1273: different `tracking_issue` than before

[alexcrichton]

Thanks @stjepang! r=me other than the tidy error

[ghost]

The tidy errors have been fixed.

[Mark-Simulacrum]

Travis appears to still be failing.

[ghost]

All right, should be good to go now. :)

[Mark-Simulacrum]

@bors r=alexcrichton

[bors]

📌 Commit 66f8cdd has been approved by alexcrichton

[bors]

⌛ Testing commit 66f8cdd with merge 1d2db7b...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 1d2db7b to master...