Rollup merge of #135847 - edwloef:slice_ptr_rotate_opt, r=scottmcm

optimize slice::ptr_rotate for small rotates

r? `@scottmcm`

This swaps the positions and numberings of algorithms 1 and 2 in `slice::ptr_rotate`, and pulls the entire outer loop into algorithm 3 since it was redundant for the first two. Effectively, `ptr_rotate` now always does the `memcpy`+`memmove`+`memcpy` sequence if the shifts fit into the stack buffer.
With this change, an `IndexMap`-style `move_index` function is optimized correctly.

Assembly comparisons:
- `move_index`, before: https://godbolt.org/z/Kr616KnYM
- `move_index`, after: https://godbolt.org/z/1aoov6j8h
- the code from `#89714`, before: https://godbolt.org/z/Y4zaPxEG6
- the code from `#89714`, after: https://godbolt.org/z/1dPx83axc

related to #89714
some relevant discussion in https://internals.rust-lang.org/t/idea-shift-move-to-efficiently-move-elements-in-a-vec/22184

Behavior tests pass locally. I can't get any consistent microbenchmark results on my machine, but the assembly diffs look promising.

Author: Zalathar