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]

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 ⚠️
src/emmy/calculus/derivative.cljc	81.81%	1 Missing and 1 partial ⚠️
src/emmy/abstract/function.cljc	91.66%	1 Missing ⚠️
src/emmy/series.cljc	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                 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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