Remove Unused Project Includes and Forward-Declare Where a Type Is Only Referenced

Generated with include-what-you-use and applied conservatively. Only OrcaSlicer's own headers, the ones under src/ and tests/, are removed or forward-declared; standard-library and third-party includes are left alone. An include is removed only when both the Release and the Debug configuration leave it unused, never from inside a conditional block, and never from a file with platform-specific blocks, which only gain includes. Files whose only use of a header sits behind a feature or debug macro (libvgcode's OpenGL ES and marker code, the ARACHNE/TESTS_EXPORT_SVGS debug output) keep their includes.

clonable_ptr.hpp gains #pragma once; it had no include guard and was only safe while Config.hpp was its sole includer.

src/libslic3r/Arrange.cpp

@@ -45,6 +45,7 @@
 #include <boost/log/trivial.hpp>
 #include <boost/multiprecision/integer.hpp>
 #include <boost/rational.hpp>
+#include "MultiMaterialSegmentation.hpp"
 
 namespace libnest2d {
 #if !defined(_MSC_VER) && defined(__SIZEOF_INT128__) && !defined(__APPLE__)