Stop Format/STEP.hpp Defining a Global fs Alias

Every file that included STEP.hpp, directly or not, got namespace fs = boost::filesystem at global scope, and 29 sources and three headers relied on it without saying so. Headers now spell out boost::filesystem, and each source that uses fs declares the alias itself.

src/slic3r/GUI/WebGuideDialog.cpp

@@ -79,6 +79,8 @@
 #include <libslic3r/Utils.hpp>
 #include "CreatePresetsDialog.hpp"
 
+namespace fs = boost::filesystem;
+
 using namespace nlohmann;
 
 namespace Slic3r { namespace GUI {