Replace a few more deprecated pre-C++11 styles

[aroffringa]

The functions std::binary_function and std::mem_fun have been deprecated in C++11, and are removed in C++20. This pull request replaces them with more modern code. Together with #1889 this removes all warnings on recent compilers. This one is fairly small.

[kripton]

Wow, the syntax in StringUtils.cpp needs a bit getting used to. On the other hand, what we had before also was not that easy to read. I'd say: As long as this doesn't break stuff, reducing compiler warnings is a good thing!