Better handling of vertical spacing

[phoe]

CL-USER> (draw-cons-tree:draw-tree '((1 2) nil nil (8)))
[o|o]---[o|o]---[o|o]---[o|/]
 |       |       |       |      
 |      NIL     NIL     [o|/]
 |                       |      
 |                       8      
 |      
[o|o]---[o|/]
 |       |      
 1       2      
NIL

The (1 2) could be drawn two lines higher, since there's still space for it to fit:

[o|o]---[o|o]---[o|o]---[o|/]
 |       |       |       |      
 |      NIL     NIL     [o|/]
 |                       |      
[o|o]---[o|/]            8      
 |       |      
 1       2