unrdered()

[Brian Goetz]

At one point, we had an unordered() op.  I think it may be time to bring 
it back.

There are a growing number of ops that have optimized implementations 
for unordered streams:

  - distinct can be implemented with concurrent insertion into a CHS 
instead of merging if we don't care about order.  Not only is this less 
work (merging is expensive), but it makes distinct lazy (elements can 
flow through immediately once they've not been found in the CHS, instead 
of waiting for all the elements to be seen.)

  - sorted is non-stable in unordered streams.

  - limit/subsequence are far lighter for unordered streams (and can 
similarly be made lazy)

So a way of saying "I know you think this stream has ordering, but I 
don't care about it" is a way of opting into these optimizations.

Implementation is trivial.

Adding .unordered() could also enable us to get rid of 
.collectUnordered(), and allow more of the reduce-like ops to benefit 
from the embrace of "disorder" without API explosion.