Attach a modified flag to a value and whenever the value is accessed via get_mut() this flag will be set until saved() is called.
Technical implements Deref and DerefMut to access the value.
use crate::modify::*;
// create new Modify with a 42 in it
let mut value = Modify::new(42);
assert_eq!(value.is_modified(), false);
// set the value to 43 and check modified flag
*value = 43;
assert_eq!(value.is_modified(), true);
// reset modified flag check modified flag again
value.saved();
assert_eq!(value.is_modified(), false);