Use the safe bool ideom instead of providing operator bool

We should use the [safe bool ideom](http://www.artima.com/cppsource/safebool.html) instead of defining operator bool, as operator bool allows an implicit conversion to int, which can cause unexpected behaviour.

[re-usable safe-bool implementation](http://www.artima.com/cppsource/safebool3.html)

Search the repository for definitions of operator bool and read the surrounding driver code to understand how those conversions are used. Compare the existing behavior with the linked safe-bool implementation; done means the relevant drivers use safe-bool behavior without allowing an implicit conversion to int, with existing tests still passing.

Written by the indexing model from the issue text.