Replace some unsafe system executor code with safe code

[JoJoJet]

Objective

The function SyncUnsafeCell::from_mut returns &SyncUnsafeCell<T>, even though it could return &mut SyncUnsafeCell<T>. This means it is not possible to call get_mut on the returned value, so you need to use unsafe code to get exclusive access back.

Solution

Return &mut Self instead of &Self in SyncUnsafeCell::from_mut. This is consistent with my proposal for UnsafeCell::from_mut: rust-lang/libs-team#198.

Replace an unsafe pointer dereference with a safe call to get_mut.

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Changelog

• The function bevy_utils::SyncUnsafeCell::get_mut now returns a value of type &mut SyncUnsafeCell<T>. Previously, this returned an immutable reference.

Migration Guide

The function bevy_utils::SyncUnsafeCell::get_mut now returns a value of type &mut SyncUnsafeCell<T>. Previously, this returned an immutable reference.