Simplify ScalarFnPlugin

[gatesn]

This fix got lost in my last PR, it simplifies the plugin to be a simply dyn trait, rather than requiring an implementation around a VTable. This allows us to implement logic to deserialize deprecated encodings into new encodings.

[codspeed-hq]

Merging this PR will degrade performance by 14.32%

❌ 1 regressed benchmark
✅ 953 untouched benchmarks
⏩ 1466 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	chunked_opt_bool_canonical_into[(10, 1000)]	1.4 ms	1.6 ms	-14.32%

---

_{Comparing ngates/simplify-plugin (c12789e) with develop (afd1c36)}

Footnotes

1. 1466 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩