Handle shared structure if *PRINT-CIRCLE* is true

[phoe]

Currently:

CL-USER> (draw-cons-tree:draw-tree '(#1=(1 2 3) #1#))
[o|o]---[o|/]
 |       |      
 |      [o|o]---[o|o]---[o|/]
 |       |       |       |      
 |       1       2       3      
 |      
[o|o]---[o|o]---[o|/]
 |       |       |      
 1       2       3      
NIL

I think that this library could be modified to use reader labels for drawing conses with shared structure if *print-circle* is set to true. For example:

CL-USER> (draw-cons-tree:draw-tree '(#1=(1 2 3) #1#))
[o|o]---[o|/]
 |       |      
 |      #1#   
 |     
#1=[o|o]---[o|o]---[o|/]
    |       |       |      
    1       2       3      
NIL

Or, alternatively, whichever is easier:

CL-USER> (draw-cons-tree:draw-tree '(#1=(1 2 3) #1#))
[o|o]---[o|/]
 |       |      
#1#     #1=[o|o]---[o|o]---[o|/]
            |       |       |      
            1       2       3      
NIL

This also means that the library fails at drawing cyclic structures, such as #1=(#1#). I imagine it could be handled like this:

#1=[o|/]
    |
    #1#

Would you accept a PR that implements this?