James Gill 01ac5a97c9 Stabilize slice_select_nth_unstable
This stabilizes the functionality in slice_partition_at_index,
but under the names `select_nth_unstable*`.  The functions
`partition_at_index*` are left as deprecated, to be removed in
a later release.

Closes #55300
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..
2020-10-12 00:07:41 -04:00
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