looping

`for` loops only support looping over iterable things, not explicit
looping.  maybe there is support for something like `(0..10).step(2)` or
similar.  `while` exists as well.  for infinite loops `loop` should be
used to help the compiler along.

rust/book/src/main.rs

    print_num_type(i);
    print_num_type(f);
    print_num_type(n);
    // loops
    for x in 0..10 {
        println!("{}", x);
    }
}