Add custom on_stop callback to thread-pool, executed by each thread before joining them.

This is similar to a change that was made a while ago ( https://github.com/gabime/spdlog/pull/208 ).

Fix warning for gcc, old style cast

Fix usage of ranges and to_hex in the same compile unit

When trying to use spdlog/fmt/bin_to_hex.h in the same compile unit as spdlog/fmt/bundled/ranges.h you got a compile error because there was a multiple definitions for iterable classes. This fix renames the begin() and end() getters in dump_info into getBegin()/getEnd() in order to avoid this collision.

Added an example of ranges in example.cpp to show that it actually works (an to_hex example was already there)