move exercises, lecture examples, labs into subfolders

Exercises/C4/difficult_to_read_code.c

@@ -0,0 +1,4 @@
+#include <stdio.h>
+int main(void) { int revenue = 80; int cost = 50; int roi; 
+roi = (100 * (revenue - cost)) / cost; if (roi >= 0) { 
+printf ("%d\n", roi); } return 0; }