Fix series freeze impl

[sritchie]

• Fix series freeze impl #166 fixes a broken freeze implementation for series. Before this fix,
  sin-series and cos-series printed as these two strings, respectively:

  "(+ (expt _ 0) (* (/ -1 6) (expt _ 1)) ...)"
  "(+ (expt _ 0) (* (/ -1 2) (expt _ 1)) ...)"

  this happened because zeros were dropped and then all remaining coefficients
  were zipped with x^0, x^1 etc.

  These now print correctly as

  "(+ (expt _ 1) (* (/ -1 6) (expt _ 3)) ...)"
  "(+ (expt _ 0) (* (/ -1 2) (expt _ 2)) ...)"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.71%. Comparing base (d354463) to head (0d43257).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #166      +/-   ##
==========================================
+ Coverage   87.68%   87.71%   +0.03%     
==========================================
  Files          99       99              
  Lines       15862    15863       +1     
  Branches      853      852       -1     
==========================================
+ Hits        13908    13914       +6     
+ Misses       1101     1097       -4     
+ Partials      853      852       -1     

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