refactor: IWYU applied to the source files

`alternative_namespaces` examples left out as I consider them a subject for removal in the nearest future.

src/core/include/units/bits/external/fixed_string.h

@@ -22,9 +22,12 @@
 
 #pragma once
 
-#include <units/bits/external/hacks.h>
+// IWYU pragma: begin_exports
 #include <cstdlib>
 #include <compare>
+// IWYU pragma: end_exports
+
+#include <cstddef>
 
 // TODO use <algorithm> when moved to C++20 modules (parsing takes too long for each translation unit)
 

src/core/include/units/bits/external/type_list.h

@@ -22,7 +22,9 @@
 
 #pragma once
 
+#include <units/bits/external/hacks.h>
 #include <units/bits/external/type_traits.h>
+#include <cstddef>
 
 #ifdef _MSC_VER
 #pragma warning (push)