Stabilize slice_partition_at_index

[jagill]

This stabilizes slice_partition_at_index, including renaming partition_at_index* -> select_nth_unstable*.

Closes #55300

r? @Amanieu

[rust-highfive]

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[scottmcm]

Waiting on FCP from #55300 (comment) to finish; expected 2020-10-09.

[Amanieu]

CI seems to be failing.

[Amanieu]

Could you mark these as deprecated with a suggestion to use select_nth_unstable instead?

[jagill]

I marked them as deprecated from version 1.49.0, which is the same as the new functions land. Tell me if there’s a better way!

[Amanieu]

Thanks!

@bors r+

[bors]

📌 Commit 01ac5a9 has been approved by Amanieu

[bors]

⌛ Testing commit 01ac5a9 with merge ec40181...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: Amanieu
Pushing ec40181 to master...

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##[command]git remote add origin https://github.com/rust-lang-ci/rust
##[command]git config gc.auto 0
##[command]git config --get-all http.https://github.com/rust-lang-ci/rust.extraheader
##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/*
##[error]fatal: could not read Username for 'https://github.com': terminal prompts disabled
##[warning]Git fetch failed with exit code 128, back off 2.331 seconds before retry.
##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/*
##[error]fatal: could not read Username for 'https://github.com': terminal prompts disabled
##[error]fatal: the remote end hung up unexpectedly
##[warning]Git fetch failed with exit code 128, back off 9.286 seconds before retry.
##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/*
##[error]fatal: could not read Username for 'https://github.com': terminal prompts disabled
##[error]Git fetch failed with exit code: 128
##[error]Exit code 1 returned from process: file name '/home/runner/runners/2.273.5/bin/Runner.PluginHost', arguments 'action "GitHub.Runner.Plugins.Repository.v1_0.CheckoutTask, Runner.Plugins"'.

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