Add fletcher16-gen developer util

1. I'm not convinced our fletcher16 implementation is proper
2. It is not in coreutils

So we add some basic code to do that.

This is also the first dev-util, so add some documentation to DEVEL.md.

dev-utils/m2-functions/require.c

@@ -0,0 +1,20 @@
+/*
+ * SPDX-FileCopyrightText: 2016 Jeremiah Orians
+ * SPDX-FileCopyrightText: 2018 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#include<stdio.h>
+#include<stdlib.h>
+
+void file_print(char* s, FILE* f);
+
+void require(int bool, char* error)
+{
+	if(!bool)
+	{
+		file_print(error, stderr);
+		exit(EXIT_FAILURE);
+	}
+}