Reviving #84

[mschauer]

The documentation for stirlings1 and stirlings2 indicates that they can use BigInts if provided with such as argument. However the existing functions specify n and k arguments to be Int. This patch allows support for BigInt integers without breaking type consistency of return types, and also provides for some memoization, since otherwise the functions, since they are recursive, can be slow when handling very large integers. (@wherrera10 in #84)

[codecov[bot]]

Codecov Report

Base: 96.85% // Head: 96.89% // Increases project coverage by +0.03% :tada:

Coverage data is based on head (55725ed) compared to base (d1b633b). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #126 +/- ## ========================================== + Coverage 96.85% 96.89% +0.03% ========================================== Files 7 7 Lines 700 708 +8 ========================================== + Hits 678 686 +8 Misses 22 22 ``` | [Impacted Files](https://codecov.io/gh/JuliaMath/Combinatorics.jl/pull/126?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath) | Coverage Δ | | |---|---|---| | [src/numbers.jl](https://codecov.io/gh/JuliaMath/Combinatorics.jl/pull/126/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath#diff-c3JjL251bWJlcnMuamw=) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath)

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