code review for <functional> improvements

[tap]

• std::mem_fn and std::bind have performance issues, can't be inlined, etc.
• lots of hidden gotchas to the above
• use lambdas (it is a core language feature, not an STL feature)
• std::function is awesome, but use it only when necessary (if you can use a template class or auto or something then that is better), but it does have space cost and it prevents inlining.

See video by Stephan T Lavavej from CPPCON 2015.