Add ID back to tape, fix Alexey's Amazing Bug in reverse-mode

[sritchie]

This PR:

• Fixes Alexey's Amazing Bug for our tape implementation
• Adds the id field back into TapeCell, required for the tag replacement machinery for fixing Alexey's Bug
• Adds an emmy.differential/IPerturbed implementation to TapeCell
• Fixes some old documentation referencing :in->partials, plural
• Updates emmy.tape/tag-of to return nil in case of a non-tape argument vs ##-Inf, again preparation for forward/reverse interactions
• Adds support for partial derivatives via a selectors argument to gradient
• Fixes a bug in the g/abs implementation for TapeCell, where (g/abs <tape>) would return a positive primal
• Adds a simplify implementation for TapeCell

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.81443% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 87.68%. Comparing base (385d3fb) to head (24b89a8).

Files	Patch %	Lines
src/emmy/tape.cljc	93.81%	4 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #165 +/- ## ========================================== - Coverage 87.69% 87.68% -0.02% ========================================== Files 99 99 Lines 15811 15862 +51 Branches 851 852 +1 ========================================== + Hits 13866 13908 +42 - Misses 1094 1102 +8 - Partials 851 852 +1 ```

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