std.parallel_algorithm

[dsimcha]

Here's some very early work in progress on std.parallel_algorithm:

https://github.com/dsimcha/parallel_algorithm/blob/master/parallel_algorithm.d

I haven't compiled the docs yet because it's too much of a WIP and 
doesn't even work without a few modifications I made to std.parallelism. 
  So far I've got parallel merge, sort and dot product.

One issue is that most of these algorithms require pretty fine grained 
parallelism and the break even point depends on a lot of things, like 
the details of the hardware.  How should we go about documenting/solving 
this?  Is it reasonable to always just punt the issue of finding the 
break even point and whether one is above or below it to the user of the 
library?  If not, what is a reasonable solution?