Maybe unsound in new_unchecked

[lwz23]

Hello, thank you for your contribution in this project, I am scanning the unsoundness problem in rust project.
I notice the following code:

impl<'rt> RuntimeHandle<'rt> {
    // Creates a new RuntimeHandle; it's the caller's responsibility to make
    // sure that the runtime is not destroyed while objects under this runtime
    // are still being used
    pub fn new_unchecked(ptr: *mut sys::Runtime) -> Self {
        RuntimeHandle(ptr, PhantomData)
    }

If it is caller's responsibility to make responsibility to make are still being used, maybe this function should be marked as unsafe and doc it with ///SAFETY? This is more in line with rust's security requirements, In Rust, if only safe code is used that results in UB being considered unsound and violating Rust's security guarantees.

[lwz23]

ping

[laptou]

That's true, it probably should be marked as unsafe. Can you make a PR for this?