Exercise 3.55: Define a procedure
partial-sums that takes as argument a stream 
  S
 and returns the
stream whose elements are 
  
    S
    0
  
, 
  
    
      S
      0
    
    +
    
      S
      1
    
  
, 
  
    
      S
      0
    
    +
    
      S
      1
    
  
  +
  
    
      S
      2
    
    ,
    …
  
.  
For example, (partial-sums integers) should be the
stream 1, 3, 6, 10, 15, ….