Change default derivative impl to reverse mode

[sritchie]

This PR:

• changes the default implementation of g/partial-derivative to use reverse mode automatic differentiation
• adds new operators D-fwd and D-partial (kind of lame, what should these be?)

TODO:

• add test coverage back for forward-mode, how do we cover the interesting test cases?
• Add a switch between reverse and forward mode?
• tests and docs for the new operator... is this a good way?

[codecov]

Codecov Report

Attention: Patch coverage is 74.07407% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 87.29%. Comparing base (0ee0d6d) to head (17f24ec).

Files	Patch %	Lines
src/emmy/calculus/derivative.cljc	37.50%	2 Missing and 3 partials ⚠️
src/emmy/differential.cljc	33.33%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##           sritchie/proto     #170      +/-   ##
==================================================
- Coverage           87.53%   87.29%   -0.25%     
==================================================
  Files                  99       99              
  Lines               15777    15793      +16     
  Branches              850      853       +3     
==================================================
- Hits                13810    13786      -24     
- Misses               1117     1154      +37     
- Partials              850      853       +3     

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