Check overflow in multinomial

[inkydragon]

Fix #99

• Check overflow in multinomial. For large input, it will throw a OverflowError
• Add new tests for large input and special case.
• Update docs (Maybe it should be split into another pr)

julia> using Combinatorics

julia> using BenchmarkTools

julia> @benchmark multinomial(10,10,10,5) evals=2000
BenchmarkTools.Trial: 10000 samples with 2000 evaluations.
 Range (min … max):  170.350 ns … 514.200 ns  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):     178.400 ns               ┊ GC (median):    0.00%
 Time  (mean ± σ):   182.391 ns ±  17.635 ns  ┊ GC (mean ± σ):  0.00% ± 0.00%

  ▆▅▄▃▆█▇▅▄▄▄▃▃▂▁▁▁▁                                            ▂
  ███████████████████▇█▇▇▇▆▇▆▅▇▆▇▅▆▅▆▅▆▆▅▄▅▅▄▆▅▆▅▅▅▅▆▆▅▅▆▅▄▄▅▄▃ █
  170 ns        Histogram: log(frequency) by time        263 ns <

 Memory estimate: 0 bytes, allocs estimate: 0.

julia> @benchmark multinomial_pr(10,10,10,5) evals=2000
BenchmarkTools.Trial: 10000 samples with 2000 evaluations.
 Range (min … max):  170.500 ns … 557.450 ns  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):     178.500 ns               ┊ GC (median):    0.00%
 Time  (mean ± σ):   183.319 ns ±  21.158 ns  ┊ GC (mean ± σ):  0.00% ± 0.00%

  █▇▅▇█▇▆▅▅▄▄▃▂▂▂▁▁▁▁                                           ▂
  ████████████████████▇▇█▇█▆▆█▇▇▇▆▆▇▆▆▇▇▆▇▇▆▅▆▇▇▇▆▆▅▆▇▆▅▆▆▅▄▃▄▄ █
  170 ns        Histogram: log(frequency) by time        273 ns <

 Memory estimate: 0 bytes, allocs estimate: 0.

[inkydragon]

This pr mainly addresses overflow issues, perhaps I should split the document and update it to another pr?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.89%. Comparing base (85c7193) to head (bc12ba1).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #172   +/-   ##
=======================================
  Coverage   96.88%   96.89%           
=======================================
  Files           7        7           
  Lines         803      804    +1     
=======================================
+ Hits          778      779    +1     
  Misses         25       25           

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