Change default derivative impl to reverse mode

[sritchie]

• 170:

  • changes D and partial to use reverse mode automatic differentiation by default, and fixes all associated tests

  • adds emmy.generic/{zero?,one?,identity?} implementations (all false) to emmy.tape/Completed, in case some collection type tries to simplify these during reverse-mode AD

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 76.92308% with 3 lines in your changes missing coverage. Please review.

Project coverage is 87.40%. Comparing base (622ba21) to head (9ad363b).

Files	Patch %	Lines
src/emmy/dual.cljc	33.33%	0 Missing and 2 partials :warning:
src/emmy/calculus/derivative.cljc	85.71%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #170 +/- ## ========================================== - Coverage 87.67% 87.40% -0.28% ========================================== Files 100 100 Lines 15811 15812 +1 Branches 853 851 -2 ========================================== - Hits 13862 13820 -42 - Misses 1096 1141 +45 + Partials 853 851 -2 ```

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