Improve factorial(n, k) performance for BigInt

[FedericoStra]

Use in-place operations from Base.GMP.MPZ to reduce allocations.

With this change I get a massive speed-up, for instance I pass from

julia> @benchmark factorial(big"1000", big"500")
BenchmarkTools.Trial: 10000 samples with 1 evaluation.
 Range (min … max):  100.233 μs … 105.019 ms  ┊ GC (min … max):  0.00% … 80.08%
 Time  (median):     116.240 μs               ┊ GC (median):     0.00%
 Time  (mean ± σ):   259.614 μs ±   2.368 ms  ┊ GC (mean ± σ):  20.95% ±  2.33%

  █▇▇▄▂▁                                                        ▂
  ███████▇▇▇█▅▅▃▃▃▃▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▄ █
  100 μs        Histogram: log(frequency) by time        962 μs <

 Memory estimate: 192.52 KiB, allocs estimate: 3002.

to

julia> @benchmark factorial(big"1000", big"500")
BenchmarkTools.Trial: 10000 samples with 991 evaluations.
 Range (min … max):   49.491 ns … 92.862 μs  ┊ GC (min … max):  0.00% … 52.86%
 Time  (median):      54.742 ns              ┊ GC (median):     0.00%
 Time  (mean ± σ):   110.839 ns ±  2.088 μs  ┊ GC (mean ± σ):  23.97% ±  1.28%

   ▇█▆▁
  ▂█████▆▆▅▃▂▂▃▅▇▆▄▄▃▃▂▂▂▂▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ▂
  49.5 ns         Histogram: frequency by time          105 ns <

 Memory estimate: 40 bytes, allocs estimate: 2.

for an approximate 2000x performance boost.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (e6888be) 96.97% compared to head (dfb9766) 96.87%.

Files	Patch %	Lines
src/factorials.jl	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
- Coverage   96.97%   96.87%   -0.11%     
==========================================
  Files           7        7              
  Lines         728      736       +8     
==========================================
+ Hits          706      713       +7     
- Misses         22       23       +1     

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