Faster generation of Sobol samples

[devmotion]

There are a lot of avoidable allocations currently when generating Sobol samples (BTW a bit related: JuliaMath/Sobol.jl#35 - after this change probably the use of skip!/skip should be removed from QuasiMonteCarlo).

On the master branch

julia> using QuasiMonteCarlo, BenchmarkTools

julia> @benchmark QuasiMonteCarlo.sample(100, 10, SobolSample())
BenchmarkTools.Trial: 10000 samples with 3 evaluations.
 Range (min … max):  8.014 μs … 227.764 μs  ┊ GC (min … max): 0.00% … 93.73%
 Time  (median):     8.722 μs               ┊ GC (median):    0.00%
 Time  (mean ± σ):   9.241 μs ±   6.835 μs  ┊ GC (mean ± σ):  2.77% ±  3.59%

   ▂▄▅▅▅▆▇█▇▇▇▆▄▃▂▂▁▁▁▁▂▂▂▃▃▃▂▁▂▁▁▁▁                          ▃
  ▆██████████████████████████████████████▇▇▇▇▇▇▇███▇██▇▇▇▆▆▆▅ █
  8.01 μs      Histogram: log(frequency) by time      12.3 μs <

 Memory estimate: 26.02 KiB, allocs estimate: 124.

This PR

julia> using QuasiMonteCarlo, BenchmarkTools

julia> @benchmark QuasiMonteCarlo.sample(100, 10, SobolSample())
BenchmarkTools.Trial: 10000 samples with 7 evaluations.
 Range (min … max):  4.542 μs … 313.673 μs  ┊ GC (min … max): 0.00% … 97.50%
 Time  (median):     4.935 μs               ┊ GC (median):    0.00%
 Time  (mean ± σ):   5.396 μs ±   9.654 μs  ┊ GC (mean ± σ):  6.38% ±  3.51%

           ▁█▅▇                                                
  ▂▄▆▄▃▂▂▂▆█████▆▄▄▃▂▂▂▁▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▁▂▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ▂
  4.54 μs         Histogram: frequency by time        6.47 μs <

 Memory estimate: 10.23 KiB, allocs estimate: 14.