Create `AbstractNode` super type

[MilesCranmer]

This will make it easier to add custom metadata to trees – simply create a new type that inherits from AbstractNode with whatever fields are needed.

[github-actions[bot]]

Benchmark Results

	master	4f621ecf6037bf...	t[master]/t[4f621ecf6037bf...]
eval/ComplexF32/evaluation	13.2 ± 0.92 ms	13.2 ± 0.87 ms	1
eval/ComplexF64/evaluation	15.7 ± 1.1 ms	16 ± 1.2 ms	0.98
eval/Float32/derivative	20.6 ± 2.6 ms	20.7 ± 3.6 ms	0.995
eval/Float32/derivative_turbo	22.2 ± 2.7 ms	22.6 ± 3.5 ms	0.983
eval/Float32/evaluation	4.1 ± 0.35 ms	4.13 ± 0.32 ms	0.992
eval/Float32/evaluation_turbo	1.3 ± 0.12 ms	1.3 ± 0.11 ms	1
eval/Float64/derivative	27.9 ± 0.94 ms	28.9 ± 1.1 ms	0.964
eval/Float64/derivative_turbo	30.8 ± 1.2 ms	0.0318 ± 0.0014 s	0.971
eval/Float64/evaluation	4.21 ± 0.33 ms	4.28 ± 0.34 ms	0.985
eval/Float64/evaluation_turbo	1.74 ± 0.16 ms	1.78 ± 0.13 ms	0.979
time_to_load	1.07 ± 0.0077 s	1.06 ± 0.01 s	1.01
utils/combine_operators	0.073 ± 0.0049 ms	0.0713 ± 0.0046 ms	1.02
utils/convert/break_sharing	0.0527 ± 0.0018 ms	0.0532 ± 0.002 ms	0.991
utils/convert/preserve_sharing	0.268 ± 0.0052 ms	0.269 ± 0.0047 ms	0.996
utils/copy/break_sharing	0.0524 ± 0.0019 ms	0.0525 ± 0.0023 ms	0.998
utils/copy/preserve_sharing	0.269 ± 0.0051 ms	0.271 ± 0.0052 ms	0.996
utils/count_constants	0.0317 ± 0.0008 ms	0.033 ± 0.0009 ms	0.961
utils/count_depth	0.0333 ± 0.0008 ms	0.0322 ± 0.0008 ms	1.03
utils/count_nodes	31.3 ± 0.8 μs	31.2 ± 0.8 μs	1
utils/get_set_constants!	0.129 ± 0.0024 ms	0.125 ± 0.0027 ms	1.03
utils/has_constants	9.37 ± 0.62 μs	9.76 ± 0.66 μs	0.96
utils/has_operators	2.84 ± 0.056 μs	2.87 ± 0.078 μs	0.992
utils/index_constants	0.101 ± 0.0047 ms	0.0993 ± 0.0049 ms	1.02
utils/is_constant	11.2 ± 0.74 μs	11.9 ± 0.86 μs	0.938
utils/simplify_tree	0.2 ± 0.019 ms	0.203 ± 0.019 ms	0.983
utils/string_tree	0.392 ± 0.0089 ms	0.392 ± 0.0098 ms	1

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 5997041598

• 31 of 31 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 91.1%

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Totals
Change from base Build 5848943298:	0.03%
Covered Lines:	1085
Relevant Lines:	1191

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💛 - Coveralls