Prefer core::ffi::c_void type over libc::c_void

[Lokathor]

All c_void are in some sense similar since you can always just cast between them, however, since core offers a c_void type we should at least use that common point of communication as much as possible.

[Osspial]

I'm not opposed to doing this, but it's worth noting that we'll still have to pull in libc to maintain no_std compatibility on Linux, since X11 uses a c_ulong to represent the window handle.