Mention counting evaluations as an alternative to wall-clock time

[Chris Warburton]

Wall-clock time has some drawbacks. For example, it depends on the
machine being used, the programming language, etc.

Jianguo mentioned that 'number of expressions evaluated' could be an
alternative, which is less dependent on the implementation. We can
mention this possibility, and the reason we avoided it (difficult to
measure for existing tools; mostly IsaCoSy).