Start relying on randomkit and slowly deprecate use of leesper/go_rng

[chewxy]

This is a profile from a recent NN run (resnet + extras)

Showing top 10 nodes out of 55
      flat  flat%   sum%        cum   cum%
    83.61s 21.93% 21.93%     83.65s 21.95%  runtime.cgocall
    54.80s 14.38% 36.31%     55.81s 14.64%  gorgonia.org/gorgonia.(*maxPoolOp).f64s
    26.68s  7.00% 43.31%    173.86s 45.61%  github.com/leesper/go_rng.UniformGenerator.Float64Range
    22.25s  5.84% 49.15%     22.25s  5.84%  sync.(*Mutex).Lock
    21.34s  5.60% 54.75%     21.34s  5.60%  sync.(*Mutex).Unlock
    20.72s  5.44% 60.18%     20.72s  5.44%  gonum.org/v1/gonum/internal/asm/f64.AxpyUnitaryTo
    17.27s  4.53% 64.71%    193.17s 50.68%  gorgonia.org/gorgonia.Uniform64
    16.04s  4.21% 68.92%    147.18s 38.61%  github.com/leesper/go_rng.UniformGenerator.Float64
    14.33s  3.76% 72.68%     14.91s  3.91%  runtime.newdefer
    13.30s  3.49% 76.17%     37.43s  9.82%  math/rand.(*Rand).Float64
(pprof) web

The reason why leespe/go_rng is slow is due to mutex locking (and reliance on the math/rand package. A better idea is to use a PCG random number generator (h/t: @dgryski) , which is what gorgonia.org/randomkit aims to be.