Global: Change <iostream> include into <ostream>

<iostream> injects a static constructor into every translation unit that
includes the header--even if nothing from the header is used. This can
result in minor initial program slowdown, as all of these constructors
need to run before main() can execute.

Instead, we can use <ostream>, which includes all of the necessary
machinery that we need.

include/athena/Global.hpp

@@ -1,6 +1,6 @@
 #pragma once
 
-#include <iostream>
+#include <ostream>
 #include "athena/Types.hpp"
 
 #define FMT_STRING_ALIAS 1

src/athena/Global.cpp

@@ -2,6 +2,7 @@
 #include "athena/Utility.hpp"
 #include <cstdio>
 #include <cstdlib>
+#include <iostream>
 
 #define FMT_STRING_ALIAS 1
 #define FMT_ENFORCE_COMPILE_STRING 1