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/libslic3r/GCode.cpp

@@ -133,6 +133,8 @@ using namespace std::literals::string_view_literals;
 
 #include <assert.h>
 
+namespace fs = boost::filesystem;
+
 namespace Slic3r {
 
     //! macro used to mark string used at localization,