Replace Differential implementation with simplified Dual (2/N)

[sritchie]

This PR

• replaces The Differential generalized dual and its term list algebra with a simplified Dual number type
• deletes emmy.util.vector-set and tests, as these are no longer used
• adds a nil implementation for extract-tangent, meaning that nil-valued functions now work with D

This new approach works because the emmy.differential/*active-tags* stack allows us to make sure that lifted binary operations always wrap their output in a new Dual with the tag assigned by the inner-most derivative call.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.89041% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 87.68%. Comparing base (cc47e69) to head (4499c63).

Files	Patch %	Lines
src/emmy/differential.cljc	96.61%	2 Missing and 2 partials :warning:
src/emmy/abstract/function.cljc	91.30%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #163 +/- ## ========================================== - Coverage 87.75% 87.68% -0.07% ========================================== Files 100 99 -1 Lines 15982 15811 -171 Branches 856 852 -4 ========================================== - Hits 14025 13864 -161 + Misses 1101 1095 -6 + Partials 856 852 -4 ```

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