move exercises and lectures into subfolders

Exercises/C3/hello_world.c

@@ -0,0 +1,10 @@
+#include <stdlib.h>
+#include <stdio.h>
+ 
+/* My first program */
+int main(void)
+{
+  // This is a single line of comment
+  printf("Hello World\n");
+  return 0;
+}