title: Simple way to find binary length of cpp_int
body: <p>What is a simplest way to find how many bits an arbitrary precision <code>cpp_int</code> is?
  For example 300 can be represented with a minimum of 9 bits. </p>
  
  <p>I imagine an algorithm that repeatedly right shifts until the value reaches 0, I was wondering if there was a simpler solution. </p>