hierarchy-print doesn't allow sending indent-string arg to hierarchy-map

[mousebot <mousebot@riseup.net>]

hi emacs,

`hierarchy-labelfn-indent' takes an optional second argument `indent-string'.

`hierarchy-print' calls `hierarchy-labelfn-indent', but doesn't allow providing it with an `indent-string' arg.

it also provides no second argument to the labelfn arg of `hierarchy-labelfn-indent'.
  
hierarchy-print looks like this:

```
(defun hierarchy-print (hierarchy &optional to-string)
   "Insert HIERARCHY in current buffer as plain text.

Use TO-STRING to convert each element to a string.  TO-STRING is
a function taking an item of HIERARCHY as input and returning a
string.  If nil, TO-STRING defaults to a call to `format' with \"%s\"."
   (let ((to-string (or to-string (lambda (item) (format "%s" item)))))
     (hierarchy-map
      (hierarchy-labelfn-indent (lambda (item _)
                                  (insert (funcall to-string item) "\n")))
      hierarchy)))
```

i'm working on a client and i'm using a copy of `hierarchy-print' that looks like this:

```
(defun hierarchy-print (hierarchy &optional to-string indent-string)
   "Insert HIERARCHY in current buffer as plain text.

Use TO-STRING to convert each element to a string.  TO-STRING is
a function taking an item of HIERARCHY as input and returning a
string.  If nil, TO-STRING defaults to a call to `format' with \"%s\"."
   (let ((to-string (or to-string (lambda (item) (format "%s" item)))))
     (hierarchy-map
      (hierarchy-labelfn-indent (lambda (item indent) ; add indent
                                  (insert (funcall to-string item indent) "\n")) ; add indent
                                indent-string) ; add indent-string
      hierarchy)))
```

i provide optional arg `indent-string' and hand it to `hierarchy-map'. i also provide a second 'indent' arg to the lambda that is the first arg for `hierarchy-labelfn-indent', indent is then given as the last arg of `funcall'. this allows me to print indented tree structures, which i was unable to achieve otherwise.

is there a reason this isn't the way `hierarchy-print' is coded? is there perhaps another way to achieve the same result? i have little experience with `hierarchy.el'.

regards,
m