Add safe fallbacks of pow for integer exponents and complex numbers

[devmotion]

This PR fixes #76 and the following unfortunate behaviour on the master branch:

julia> using NaNMath

julia> -1^2
1

julia> NaNMath.pow(-1, 2)
1.0

julia> 2^2
4

julia> NaNMath.pow(2, 2)
4.0

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.15%. Comparing base (20bd888) to head (f513cbf).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #83      +/-   ##
==========================================
- Coverage   97.05%   96.15%   -0.91%     
==========================================
  Files           1        1              
  Lines         102      104       +2     
==========================================
+ Hits           99      100       +1     
- Misses          3        4       +1     

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