[plt-scheme] 2htdp/image: above and friends

I'd like to suggest that above and friends from the 2htdp/image library 
change from:

(above i1 i2 is ...) -> image?
   i1 : image?
   i2 : image?
   is : image?

to:

(above is ...) -> image?
   is : image?

This is consistent with ISL's +, and it's important for the analogy 
between numbers and images that the operations remain consistent.

Also, above (as is) and apply won't work together, which I think was one 
reason for ISL moving to its current + contract.

The new function would obey:

(equal? (above i) i)
(equal? (above) (circle 0 'solid 'red))

[Maybe an empty-image binding for that latter value should be in 
included in the library?]

David