Reorganize

- Extracted functions to //lib:test for writing tests

- Extracted functions to lib/util.h if they are useful outside tests

lib/util.c

@@ -0,0 +1,26 @@
+#include "lib/util.h"
+
+#include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+void Fatalf(const char *msg, ...)
+{
+	va_list ap;
+	fputs("Error: ", stderr);
+	va_start(ap, msg);
+	vfprintf(stderr, msg, ap);
+	va_end(ap);
+	fputc('\n', stderr);
+	exit(1);
+}
+
+static const char *const kErrorNames[] = {"ok", "no memory", "bad data"};
+
+const char *ErrorDescription(ErrorCode err)
+{
+	if (err < 0 || (int)(sizeof(kErrorNames) / sizeof(*kErrorNames)) <= err) {
+		return NULL;
+	}
+	return kErrorNames[err];
+}