feat: freestanding support added

Resolves #564, #565, and #556

src/core/mp-units-core.cpp

@@ -8,8 +8,11 @@ export module mp_units.core;
 
 #include <mp-units/compat_macros.h>
 #include <mp-units/concepts.h>
-#include <mp-units/format.h>
 #include <mp-units/framework.h>
+
+#if MP_UNITS_HOSTED
+#include <mp-units/format.h>
 #include <mp-units/math.h>
 #include <mp-units/ostream.h>
 #include <mp-units/random.h>
+#endif