adding arena (p2)

tests/cpp_build_test.cpp

@@ -3,11 +3,13 @@
 
 int main(void)
 {
+    MD_Arena *arena = MD_ArenaNew(1ull << 40);
+    
     printf("%d\n", MD_CPP_VERSION);
     
     MD_String8 foo = "foo"_md;
     MD_String8 bar = "bar"_md;
-    MD_String8 str = MD_S8Fmt("%S%S", foo, bar);
+    MD_String8 str = MD_S8Fmt(arena, "%S%S", foo, bar);
     printf("%.*s", MD_S8VArg(str));
     return 0;
 }