Fix series freeze impl

[sritchie]

• 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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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