Merge dual number + tape implementations?

[sritchie]

It turns out we can accumulate forward-mode as we go in the forward pass of reverse mode... adding this here since it works, AND it confuses me!!

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.25954% with 18 lines in your changes are missing coverage. Please review.

Project coverage is 87.31%. Comparing base (556cc83) to head (d3a16ba).

Files	Patch %	Lines
src/emmy/tape.cljc	84.94%	11 Missing and 3 partials :warning:
src/emmy/calculus/derivative.cljc	81.81%	1 Missing and 1 partial :warning:
src/emmy/abstract/function.cljc	91.66%	1 Missing :warning:
src/emmy/series.cljc	92.30%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## sritchie/nested #161 +/- ## =================================================== - Coverage 87.46% 87.31% -0.16% =================================================== Files 100 100 Lines 16100 16188 +88 Branches 860 866 +6 =================================================== + Hits 14082 14134 +52 - Misses 1158 1188 +30 - Partials 860 866 +6 ```

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