Implement Sync + Send for HWND

[Ciantic]

Suggestion

HWND used to be a wrapper of isize, like HWND(pub isize), this meant it was also Send+Sync.

Now that HWND is:

pub struct HWND(pub *mut core::ffi::c_void);

It means it's not automatically Sync + Send. I don't see why it couldn't be Sync+Send because HWNDs are single-use values, and multiple threads and processes already use them.

My suggestion is to do this:

unsafe impl Sync for HWND {}
unsafe impl Send for HWND {}

Maybe for other handles as well?

[tim-weis]

This has been discussed in #3169. In a nutshell, handles are used in too many different ways to mark them all as Send and Sync. As an example, DestroyWindow cannot be called from a thread other than the window's owning thread.