Fix power (^) function gives NaN at 0

[tansongchen]

This solves #54 #41 and #33 by eliminating division altogether

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (6b0b0e9) 85.48% compared to head (748d7db) 84.46%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
- Coverage   85.48%   84.46%   -1.03%     
==========================================
  Files           6        6              
  Lines         248      251       +3     
==========================================
  Hits          212      212              
- Misses         36       39       +3     

Files	Coverage Δ
src/codegen.jl	100.00% <100.00%> (ø)
src/primitive.jl	85.71% <85.71%> (-2.71%)	⬇️

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