Implement UnwindSafe on core

[taiki-e]

We can do the same thing as smol-rs/event-listener#78.

concurrent-queue/src/lib.rs

Lines 102 to 105 in cbdf9e8

	#[cfg(feature = "std")]
	impl<T> UnwindSafe for ConcurrentQueue<T> {}
	#[cfg(feature = "std")]
	impl<T> RefUnwindSafe for ConcurrentQueue<T> {}

concurrent-queue/Cargo.toml

Line 13 in cbdf9e8

rust-version = "1.59"