- unified used exception types
- removed any camel case names

Keyword 'inline' is useless when member functions are defined in headers

See https://isocpp.org/wiki/faq/inline-functions#inline-member-fns-more

Remove useless typename

Avoid a warning with GCC 4.9:

son.hpp:2361:49: warning: declaration 'struct std::bidirectional_iterator_tag' does not declare anything
         using iterator_category = typename std::bidirectional_iterator_tag;
                                                 ^

Avoid warning with Xcode's clang

94de11e, f673c8d, 540b763

 Fix typos

Replace `default_callback` function with `nullptr` and check for null…

Replace `default_callback` function with `nullptr` and check for null callback function before calling it. This is cleaner and better performing.