"Performance" fixes suggested by cppcheck

"Performance" fixes suggested by cppcheck

While cppcheck did not turn out any suspicious error messages, using
the "performance" flag highlighted several nitpicks in three categories
 * do not use it++ for iterators, ++it is better
 * do not use size() to test for emptyness, empty() is here
 * do not use "const T" as a function parameter, "const & T" is better

I doubt that any of these is a real performance problem, but the code is cleaner anyway.