fast partition

[pepijndevos]

Implements suggestions, fixing #39

Current master:
#39 (comment)

On this branch, which is similar to SlidingWindow with support for arbitrary step sizes:

julia> @btime g(1:100000)
  38.671 μs (2 allocations: 144 bytes)
(4999850001, 4999949999, 5000049997)

julia> @btime g2(1:100000)
  36.638 μs (1 allocation: 80 bytes)
(4999850001, 4999949999, 5000049997)

[oxinabox]

technically this can be breaking if someone was doing something like
dispatching on this as a type parameter of another type.

But I double anyone is so i think it's fine