slice: Mark rotate_left, rotate_right unstably const

[okaneco]

• Add the const unstable const_slice_rotate feature
• Mark <[T]>::rotate_left and <[T]>::rotate_right as const unstable

The internal rotate functions use <*mut T>::replace and ptr::swap_nonoverlapping which were const-stabilized in 1.88.

Two changes were needed in the rotate.rs module to make these functions const:

1. A usage of cmp::min was replaced with a local function implementation of Ord::min.
2. A for start in 1..gcd loop was changed to a while loop with an increment variable.

This needs libs-api approval and cc-ing const-eval.
Tracking issue will be made upon acceptance.

[rustbot]

r? @Mark-Simulacrum

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