Improve the Marsaglia Gaussian generator

[gbaydin]

The generator for Gaussian random numbers can generate two independent samples, but we are currently discarding one of them. We can improve the code to keep the second sample in memory to return it in the next call to the generator. This should improve efficiency.

DiffSharp/src/DiffSharp.Core/Util.fs

Line 48 in f1fb160

static member Normal() =

See https://en.wikipedia.org/wiki/Marsaglia_polar_method

[gbaydin]

Note that this is only used by the reference backend.