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

[mousebot]

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