Add safe path from T: HasRawWindowHandle to TrustedHandle

[DJMcNab]

It seems to me that the requirements of TrustedHandle::new being correct are exactly that the handle matches the invariants on HasRawWindowHandle, i.e.

impl TrustedWindowHandle {
    fn from_handle<T: HasRawWindowHandle>(has_handle: &T){
        // Safe: `impl HasRawWindowHandle` requires that `raw_window_handle`
        // returns a valid handle
        unsafe { Self::new(has_handle.raw_window_handle()) };
    }
}

would be valid (and useful)

Original comment from (#52 (comment))

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Would it be possible to have a safe impl of new or equivalent for any T: HasRawWindowHandle?

Specifically, an implementation which would replace https://github.com/bevyengine/bevy/blob/a68c78cef5fb2112617de095efd4e64cba1587eb/crates/bevy_winit/src/winit_windows.rs#L117-L119

(I need this for bevyengine/bevy#1279, which is currently assuming that the impl could be correct, so I would like to know if that assumption is wrong anyway. That PR is otherwise blocked on #58 anyway)

[Lokathor]

This seems good. I might have time to work on this crate during this weekend/Monday, and if so I can do this easily.

[DJMcNab]

Excellent, I just wanted to sanity check that I wasn't missing any reason that wouldn't work.