Extend the const swap feature

[est31]

Adds the const_swap feature gate to three more swap functions. cc tracking issue #83163

impl<T> [T] {
    pub const fn swap(&mut self, a: usize, b: usize);
    pub const unsafe fn swap_unchecked(&mut self, a: usize, b: usize);
}
impl<T: ?Sized> *mut T {
    pub const unsafe fn swap(self, with: *mut T);
}

[rust-highfive]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

Can we squash commits? r=me once that's done

[est31]

@Mark-Simulacrum I've squashed them. so re-r? @Mark-Simulacrum

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit eeaa2f1 has been approved by Mark-Simulacrum