Replace mem::forget with ManuallyDrop, fixing potential UB

As shown in rust-lang/miri#1508,
it was possible to "find UB" in `replace_with` with Miri.

src/lib.rs

@@ -94,7 +94,7 @@ struct OnDrop<F: FnOnce()>(mem::ManuallyDrop<F>);
 impl<F: FnOnce()> Drop for OnDrop<F> {
 	#[inline(always)]
 	fn drop(&mut self) {
-		(unsafe { ptr::read(&*self.0) })();
+		(unsafe { mem::ManuallyDrop::take(&mut self.0) })();
 	}
 }
 
@@ -103,8 +103,8 @@ impl<F: FnOnce()> Drop for OnDrop<F> {
 pub fn on_unwind<F: FnOnce() -> T, T, P: FnOnce()>(f: F, p: P) -> T {
 	let x = OnDrop(mem::ManuallyDrop::new(p));
 	let t = f();
-	let _ = unsafe { ptr::read(&*x.0) };
-	mem::forget(x);
+	let mut x = mem::ManuallyDrop::new(x);
+	unsafe { mem::ManuallyDrop::drop(&mut x.0) };
 	t
 }