fix BigFloat pow

[oscardssmith]

fixes #93

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.15%. Comparing base (72357e3) to head (01a0d0f).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
+ Coverage   94.00%   94.15%   +0.15%     
==========================================
  Files           1        1              
  Lines         150      154       +4     
==========================================
+ Hits          141      145       +4     
  Misses          9        9              

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[andreasnoack]

Don't we do reviews here?