title: Faster way of adding negative signed to unsigned
body: <p>Assuming I have <code>a: usize</code> and a negative <code>b:isize</code> how do I achieve the following semantics - <strong>reduce a by absolute value of b</strong> in fastest manner possible? </p>
  
  <p>I already thought of <code>a - (b.abs() as usize)</code>, but I'm wondering if there is a faster way. Something with bit manipulation, perhaps?</p>