Reverse-mode AD implementation

[sritchie]

This implementation of reverse-mode AD is modeled after the approach described in the dysvunctional language docs. The current version:

• works for $R^n \to R^m$ functions
• can handle nesting of other gradient calls

From the CHANGELOG:

• 154:

  • Adds emmy.tape with an implementation of reverse-mode automatic differentiation. The implementation is based on Alexey Radul's implementation in dvl, and seems to be higher-performance by quite a bit and capable of replacing our forward-mode implementation.

  • The centerpiece of the implementation is emmy.tape/gradient, which can handle $R^n \to R^m$ functions, as well as nested derivatives.

  • All operations supported by [[emmy.differential/Differential]] are supported by the backing [[emmy.tape/TapeCell]] instance.

What we're missing:

• reverse-mode as an operator. @littleredcomputer , should this become the default, and forward-mode for partial? or should we install this under the existing Grad operator?
• proper interaction with forward-mode... we need lift-2 to be able to handle tapes interacting with differential instances.
• Support for partial derivatives? @littleredcomputer reverse-mode is bad for partials so maybe we skip or discourage this.

[codecov[bot]]

Codecov Report

Attention: 18 lines in your changes are missing coverage. Please review.

Comparison is base (e674ac5) 87.57% compared to head (c1c4a86) 87.73%.

:exclamation: Current head c1c4a86 differs from pull request most recent head eb3905b. Consider uploading reports for the commit eb3905b to get more accurate results

Files	Patch %	Lines
src/emmy/tape.cljc	94.91%	6 Missing and 9 partials :warning:
src/emmy/differential.cljc	94.82%	0 Missing and 3 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #154 +/- ## ========================================== + Coverage 87.57% 87.73% +0.16% ========================================== Files 99 100 +1 Lines 15672 15975 +303 Branches 845 851 +6 ========================================== + Hits 13725 14016 +291 - Misses 1102 1108 +6 - Partials 845 851 +6 ```

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[littleredcomputer]

Having read this twice I think it's a beautifully compact implementation of the idea. I'm eager to try to understand how it works by printing out the tape structure before the reverse process.