Key changes

• Added std::backtrace support for use in multiple modules both on Windows and Linux, both in unloading mode and without it.
  (on Windows some hooks are needed for backtraces to work correctly in multiple modules (dynamic libraries))
• relib_host::load_module is now unsafe due to backtrace support on Windows
• Added before load check (when some dll is already loaded it can't be loaded again, load_module returns error) and final unload check (when some dll failed to unload module.unload() returns error) to Windows
• Removed backtrace dependency which was used by custom panic hook