Auto merge of #114231 - ttsugriy:binary_search_slice, r=cjgillot

[rustc_data_structures] Use partition_point to find  slice range end.

This PR uses approach introduced in https://github.com/rust-lang/rust/pull/114152 to find
the end of the range. It's much easier to understand and reason about invariants of such
implementation.
Technically it's possible to make it even shorter by returning `&[start..end]` unconditionally
because even if searched item is not present in the slice, `start` and `end` would point at
the same index, so the range would be empty. The reason I decided not to use this shorter
implementation is because it would involve more comparisons in case there are no elements
in the slice with key equal to `key`.

Also, not that it matters much, but this implementation also improves perf according to the
benchmark below:
https://gist.github.com/ttsugriy/63c0ed39ae132b131931fa1f8a3dea55

The results on my M1 macbook air are:
```
Running benches/bin_search_slice_benchmark.rs (target/release/deps/bin_search_slice_benchmark-90fa6d68c3bd1298)
Benchmarking multiply add/binary_search_slice: Collecting 100 samples in estimated 5.0002 s (1
multiply add/binary_search_slice
                        time:   [44.719 ns 44.918 ns 45.158 ns]
                        No change in performance detected.
Found 3 outliers among 100 measurements (3.00%)
  1 (1.00%) high mild
  2 (2.00%) high severe
Benchmarking multiply add/binary_search_slice_new: Collecting 100 samples in estimated 5.0001
multiply add/binary_search_slice_new
                        time:   [36.955 ns 37.060 ns 37.221 ns]
                        No change in performance detected.
Found 7 outliers among 100 measurements (7.00%)
  3 (3.00%) high mild
  4 (4.00%) high severe
```

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