Rollup merge of #109409 - WaffleLapkin:progamer, r=dtolnay

Add `minmax{,_by,_by_key}` functions to `core::cmp`

This PR adds the following functions:

```rust
// mod core::cmp
#![unstable(feature = "cmp_minmax")]

pub fn minmax<T>(v1: T, v2: T) -> [T; 2]
where
    T: Ord;

pub fn minmax_by<T, F>(v1: T, v2: T, compare: F) -> [T; 2]
where
    F: FnOnce(&T, &T) -> Ordering;

pub fn minmax_by_key<T, F, K>(v1: T, v2: T, mut f: F) -> [T; 2]
where
    F: FnMut(&T) -> K,
    K: Ord;
```
(they are also `const` under `#[feature(const_cmp)]`, I've omitted `const` stuff for simplicity/readability)

----

Semantically these functions are equivalent to `{ let mut arr = [v1, v2]; arr.sort(); arr }`, but since they operate on 2 elements only, they are implemented as a single comparison.

Even though that's basically a sort, I think "sort 2 elements" operation is useful on it's own in many cases. Namely, it's a common pattern when you have 2 things, and need to know which one is smaller/bigger to operate on them differently.

I've wanted such functions countless times, most recently in #109402, so I thought I'd propose them.

----

r? libs-api

Author: matthiaskrgr