Unsound lifetime extension for DOM thread local

[Timbals]

The documentation for LocalKey states

The with method yields a reference to the contained value which cannot outlive the current thread or escape the given closure.

The current dom context is stored in a thread local and its lifetime is extended beyond, and escapes, the closure

yakui/crates/yakui-core/src/context.rs

Lines 34 to 41 in ba96329

	// SAFETY: Rust won't give us a 'static reference here because this
	// thread could send the 'static value to another thread and then die
	// prematurely. This would be bad.
	//
	// However, we have some good news! Ref<T> is not Send and never will
	// be. We can rest assured that this reference will never outlive the
	// thread it originated from.
	let context = unsafe { extend_lifetime(context) };

In practice, this can lead to a use-after-free if the current dom is accessed in the destructor of another thread local.

This is similar to rust-random/rand#968, and they fixed it by wrapping the thread local in a std::rc::Rc.
That issue also links to a test case that actually produces a use-after-free on macOS which would also apply to Yakui.

[LPGhatguy]

Cool, thank you for the note on this! I'm interested in fixing this.