Recursive directory traversal.

Example

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

main :: (fn(io : Io, exn : Exception) -> unit)({
  entries := io.await(walk(Path.new(`/tmp`), io), { io, exn });
  i := usize(0);
  while runtime((i < entries.len())), {
    e := entries(i);
    println(e.path);
    i = (i + usize(1));
  };
});