Use cmp::min() instead of const_min() helper in ptr_rotate()

[BD103]

The private ptr_rotate() function used a custom const_min() function because std::cmp::min() wasn't const. cmp::min() is now unstably const, so we can switch to that instead.

@rust-lang/wg-const-eval The rustc dev guide said to ping you all when using #[rustc_allow_const_fn_unstable(...)]. Please let me know if I used it correctly!

[rustbot]

⚠️ #[rustc_allow_const_fn_unstable] needs careful audit to avoid accidentally exposing unstable
implementation details on stable.

cc @rust-lang/wg-const-eval

[rustbot]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

I don't think this is worth an allow_unstable, so I'm going to close. When it stabilizes we can switch.