add 'raw_handle' function and impl 'send' trait for 'RegKey'

[WLBF]

I was wirting something like:

let hklm = RegKey::predef(HKEY_LOCAL_MACHINE);
let reg_key = hklm.open_subkey(r#"SOFTWARE\Microsoft\Windows\CurrentVersion"#).unwrap();

let handle = builder.spawn(move || {
    unsafe {
        let ret = RegNotifyChangeKeyValue(
            reg_key.raw_handle(),
            watch_subtree as i32,
            notify_filter,
            wait_handle.handle(),
            true as i32,
        );
        .....
    }
})?;

I thing add raw_handle function to RegKey allow us use RegKey in ffi programming. Since winapi::shared::minwindef::HKEY is a raw pointer, it should be fine to implement send trait for RegKey.

[gentoo90]

Thanks! Published to crates.io as 0.5.1.