RFC: Improve static load balancing in @parallel

This patch ensures that static load balancing is as good as possible: each
worker process gets either M or M+1 elements of the input range.  Previously
the remainder of the work would all go to the last process, which increases
the wall time by up to a factor of two.

Also fix a bug in preduce() which could cause a bounds exception when the
length of the input range was smaller than the number of workers.  It's not
clear to me that this is actually a bug in the code, or a bug in the way that
bounds checking for ranges works.  In particular the following seem to be
inconsistent:
````
julia> (1:10)[11:10]
ERROR: BoundsError()
 in getindex at range.jl:111
````
vs
````
julia> ([1:10])[11:10]
0-element Int64 Array
````