clean up the rest of the math methods

[n14little]

This Pull Request fixes/closes #514

It changes the following:

• Refactors the following methods:
  • Math::atan2
  • Math::log
  • Math::log10
  • Math::log2
  • Math::min
  • Math::pow
  • Math::sign

[codecov]

Codecov Report

Merging #523 into master will decrease coverage by 0.03%.
The diff coverage is 60.46%.

@@            Coverage Diff             @@
##           master     #523      +/-   ##
==========================================
- Coverage   68.09%   68.05%   -0.04%     
==========================================
  Files         169      169              
  Lines        9925     9932       +7     
==========================================
+ Hits         6758     6759       +1     
- Misses       3167     3173       +6     

Impacted Files	Coverage Δ
boa/src/builtins/math/mod.rs	76.07% <60.46%> (-2.78%)	⬇️

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