cumulate

[Brian Goetz]

After an offline conversation with Doug, we're considering ditching 
cumulate from Streams.  Reasons include:

1.  Everybody looks at it and says WTF?  And then has a YAGNI fit about 
throwing the kitchen sink into this API.

2.  The form in which cumulation is exposed here -- stream in, stream 
out -- is really all that useful to algorithms that need it.  It would 
be better to expose as an operation on Arrays instead.