Tidy up includes:

* Replace boost with std equivalents:
  - bind, ref, cref, function, placeholders
* More "include what you use"
* Remove unnecessary includes

beast/module/core/core.unity.cpp

@@ -232,6 +232,3 @@ void beast_reportFatalError (char const* message, char const* fileName, int line
 #pragma pop_macro("_aligned_offset_recalloc")
 #pragma pop_macro("_aligned_msize")
 #endif
-
-// Must be outside the namespace
-#include <beast/module/core/system/BoostPlaceholdersFix.cpp>