Add missing impl Fn for &mut F where F: Fn

[Kivooeo]

Currently, there are blanket implementations for &F and &mut F for FnMut and FnOnce, but not for Fn, as a result, &mut F implements fewer of the Fn traits than &F, which is inconsistent and this PR fills that coherence hole

Technically, this is a breaking change because Fn is a fundamental trait, however, practical breakage is expected to be minimal, as such overlapping impl patterns are extremely rare

pub trait Trait {}
impl<F: Fn()> Trait for F {}
impl Trait for &fn() {}     // not allowed
impl Trait for &mut fn() {} // allowed

Because of this, second impl for &mut fn() works fine but shouldn't

Fixes #147931

[rustbot]

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[Noratrieb]

@bors try

[Noratrieb]

r? libs-api

[rust-bors]

☀️ Try build successful (CI)
Build commit: ddcf048 (ddcf048784667508cd676fef4667f88ae15628a3, parent: 292be5c7c05138d753bbd4b30db7a3f1a5c914f7)

[theemathas]

Is this "since" line correct?

[saethlin]

@craterbot run mode=check-only

[craterbot]

👌 Experiment pr-148271 created and queued.
🤖 Automatically detected try build ddcf048
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🚧 Experiment pr-148271 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more