Project: better partition

[Andrei Alexandrescu via Digitalmars-d]

We should take advantage of the improved partition code I discussed at 
ACCU. Also there's a person on 
https://www.reddit.com/r/programming/comments/4jlkhv/accu_2016_keynote_by_andrei_alexandrescu/ 
discussing a simpler algorithm based on a couple of additional 
assumptions. The plan would go:

* Add a new public overload of partition() 
(https://dlang.org/phobos/std_algorithm_sorting.html#partition) that 
takes an index as a second argument. Implement partition with pivot per 
the slides.

* Use it in sort

* Benchmark, make sure it's faster.

* Yay.


Andrei