Add fast multiplicative inverses

[timholy]

This is part 1 of 2 towards a new version of ReshapedArrays (#10507). I'm making them separate PRs because part 1 is best reviewed by the "numbers" crowd, and part 2 by the "arrays" crowd. I committed this as @simonster, since the core code here was developed by him.

I benchmarked this on 4 different machines against libdivide, which claims to have emphasized performance. We're doing pretty well, especially on master:

	libdivide (g++ -O3)	julia-0.4 div	julia-0.4 fastdiv	julia-0.5 div	julia-0.5 fastdiv
core i7 L640 (Westmere)	30.4ms	114ms	40.5ms
core i7 5500U (Broadwell-U)	15.8ms	92ms	23.2ms	91ms	16.2ms
xeon ES-2650 (Sandybridge)	34.2ms	184ms	34.6ms	104ms	23.7ms
opteron 8439 SE (Istanbul)	95.7ms	180ms	53.1ms

julia-0.5 isn't installed on the machines missing numbers (one does not have enough disk space left for a separate build, and the other is Ubuntu 12.04 which has troubles building master).

The tests are in this gist: https://gist.github.com/timholy/1d21885f00ac066d7237.

[timholy]

More results on the Broadwell, for different denominators:

d	libdivide (g++ -O3)	julia-0.5 div	julia-0.5 fastdiv
1	13.2ms	88.8ms	6.5ms
2	14.2ms	91.0ms	15.7ms
3	17.0ms	91.7ms	15.8ms
4	14.1ms	91.6ms	15.7ms
5	14.4ms	91.0ms	15.7ms
6	17.0ms	91.1ms	15.7ms
7	14.4ms	91.0ms	15.7ms
8	14.2ms	91.7ms	15.8ms
9	17.1ms	90.8ms	15.7ms
10	14.2ms	91.0ms	15.7ms

[tkelman]

I wonder if these should just be methods of unsigned{T<:Signed}(::Type{T}) ?

[tkelman]

Is there an algorithmic reference for this technique? The comments here aren't especially useful at explaining how this works.

[timholy]

@tkelman review comments addressed.

I made several improvements in performance. The cost of constructing one of these objects is now equivalent to approximately 14 div operations, or about half of what it used to be. This is likely to still be quite noticeable for creating small ReshapedArrays, but I suspect this is the best we can do.

[musm]

Perhaps not fully relevant but potentially useful for other tricks: http://www.jjj.de/fxt/fxtbook.pdf