Add differential operator

[MilesCranmer]

This adds the D operator for differentiating within a TemplateStructure. It can also be used for other expression types too as a way to compute a symbolic derivative before evaluation.

[github-actions]

Benchmark Results

	master	5fd91a6...	master/5fd91a679ce556...
search/multithreading	16.9 ± 0.96 s	17.2 ± 0.91 s	0.983
search/serial	29 ± 0.65 s	28.2 ± 0.93 s	1.03
utils/best_of_sample	1.56 ± 0.3 μs	1.62 ± 0.42 μs	0.963
utils/check_constraints_x10	11 ± 2.8 μs	11.2 ± 2.9 μs	0.979
utils/compute_complexity_x10/Float64	2.11 ± 0.13 μs	2.08 ± 0.11 μs	1.01
utils/compute_complexity_x10/Int64	2.03 ± 0.11 μs	2.03 ± 0.1 μs	1
utils/compute_complexity_x10/nothing	1.48 ± 0.13 μs	1.44 ± 0.13 μs	1.03
utils/insert_random_op_x10	5.6 ± 1.8 μs	5.76 ± 1.9 μs	0.971
utils/next_generation_x100	0.353 ± 0.096 ms	0.354 ± 0.099 ms	0.999
utils/optimize_constants_x10	0.0351 ± 0.008 s	0.0348 ± 0.0081 s	1.01
utils/randomly_rotate_tree_x10	5.23 ± 0.59 μs	5.29 ± 0.59 μs	0.989
time_to_load	1.86 ± 0.0091 s	1.89 ± 0.0097 s	0.983

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).

[coveralls]

Pull Request Test Coverage Report for Build 12324139530

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 95.144%

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Files with Coverage Reduction	New Missed Lines	%
src/SymbolicRegression.jl	1	98.3%

Totals
Change from base Build 12324106409:	-0.03%
Covered Lines:	3135
Relevant Lines:	3295

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💛 - Coveralls