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

[WaffleLapkin]

This PR adds the following functions:

// 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

[dtolnay]

I like it. :) Please file a library tracking issue and update the issue number in the unstable attributes.

This will need a rebase past #110393 because ~const trait bounds have been removed from the standard library.

[dtolnay]

@bors r+

[bors]

📌 Commit 0c3e0ab has been approved by dtolnay

It is now in the queue for this repository.

[bors]

⌛ Testing commit 0c3e0ab with merge 65ea825...