By changing the 'pass by value' to be a 'pass by const reference' (and
adding some const qualifiers) the calculator can take any of the three
types.
Performance could be improved further using C++11 perfect forwarding to
pass the function on without changing a thing. I added a comment to remind
us of this.
Also added a test for passing a function and a (commented out) test for
passing a lambda.
bb87e9e628