Modified Scan Partitioner for Algorithms

[dcbdan]

With these changes, the scan partitioner now takes four functions.
The first two functions are the same as previously, they operate on a
partition and then accumulate the result. The third function now
operates on each partitioner with it's corresponding accumulated result
instead of being called once. The fourth function is just to create a
result type.

Changes accomplish the following:

• Reduce overhead of calling another partitioner inside of
  scan_partitioner
• Allow third step to be called as soon as possible instead of waiting
  for all of the accumulated items to be finished
  • allow scan algorithms to not need a temporary data structure
• allow transform_exclusive_scan to operate in place

[hkaiser]

LGTM