Make all required includes explicit

Information as seen by IWYU 0.12 + clang 9 on Linux
diff --git a/Vector.h b/Vector.h
index 428b931..3cff212 100644
--- a/Vector.h
+++ b/Vector.h
@@ -9,6 +9,8 @@
 
 #include "Object.h"
 
+#include <stdbool.h>
+
 
 #ifndef DEFAULT_SIZE
 #define DEFAULT_SIZE (-1)