Sample usage looks like this:

```
(directory:walk "/etc" (lambda (x)
                         (if (directory? x)
                             (print "Got directory: %s" x)
                           (print "Got file: %s" x)
                           )))
```

To avoid the complexity of adding this as a built-in form the
implementation creates "directory:entries" to return the contents
of the given path, recursively.

If we have a list of file/directories, and a function, we can just
use apply to do the magic.

This closes #34.