Recursive directory traversal.

Example

{ walk, WalkEntry } :: import "std/fs/walker";

main :: (fn(using(io : IO)) -> unit)({
  given(exn) : Exception = {
    throw : (fn(forall(T : Type), error: AnyError) -> T)(
      { println(error.to_string()); exit(i32(1)); }
    )
  };

  entries := io.await(walk(`/tmp`));
  i := usize(0);
  while runtime((i < entries.len())), {
    e := entries.get(i).unwrap();
    println(e.path);
    i = (i + usize(1));
  };
});