Add built-in random sampling

[MilesCranmer]

This moves all the random sampling of nodes into DynamicExpressions.jl. It also makes things more explicit with respect to sampling graph nodes. For example, should one sample a GraphNode with the same probability, or proportional to how many times it is used in upstream nodes? This allows you to choose both.

TODO:

• Add NodeSampler to docs

[github-actions]

Benchmark Results

	master	6d591ed...	t[master]/t[6d591ed...]
eval/ComplexF32/evaluation	7.42 ± 0.5 ms	7.36 ± 0.43 ms	1.01
eval/ComplexF64/evaluation	9.59 ± 0.7 ms	9.65 ± 0.7 ms	0.994
eval/Float32/derivative	11 ± 2.2 ms	10.7 ± 1.7 ms	1.03
eval/Float32/derivative_turbo	12.4 ± 2.2 ms	12.5 ± 1.8 ms	0.998
eval/Float32/evaluation	2.73 ± 0.22 ms	2.66 ± 0.23 ms	1.03
eval/Float32/evaluation_turbo	0.653 ± 0.033 ms	0.649 ± 0.035 ms	1.01
eval/Float64/derivative	14.8 ± 0.74 ms	14.4 ± 0.71 ms	1.03
eval/Float64/derivative_turbo	15.6 ± 0.64 ms	15.2 ± 0.68 ms	1.03
eval/Float64/evaluation	2.91 ± 0.25 ms	2.85 ± 0.24 ms	1.02
eval/Float64/evaluation_turbo	1.17 ± 0.062 ms	1.13 ± 0.062 ms	1.04
utils/combine_operators/break_sharing	0.0398 ± 0.0024 ms	0.0401 ± 0.0025 ms	0.991
utils/convert/break_sharing	28.8 ± 0.81 μs	28.5 ± 0.73 μs	1.01
utils/convert/preserve_sharing	0.132 ± 0.004 ms	0.13 ± 0.0033 ms	1.01
utils/copy/break_sharing	29.2 ± 0.73 μs	29.4 ± 0.67 μs	0.992
utils/copy/preserve_sharing	0.131 ± 0.0036 ms	0.128 ± 0.0031 ms	1.02
utils/count_constants/break_sharing	11.4 ± 0.22 μs	11.4 ± 0.25 μs	0.996
utils/count_constants/preserve_sharing	0.114 ± 0.0031 ms	0.113 ± 0.0026 ms	1.01
utils/count_depth/break_sharing	17.8 ± 0.48 μs	17.7 ± 0.45 μs	1.01
utils/count_nodes/break_sharing	10.2 ± 0.18 μs	10.4 ± 0.19 μs	0.975
utils/count_nodes/preserve_sharing	0.117 ± 0.0032 ms	0.116 ± 0.0032 ms	1
utils/get_set_constants!/break_sharing	0.054 ± 0.0009 ms	0.0542 ± 0.00088 ms	0.997
utils/get_set_constants!/preserve_sharing	0.327 ± 0.0075 ms	0.328 ± 0.0071 ms	0.995
utils/has_constants/break_sharing	4.61 ± 0.24 μs	4.57 ± 0.22 μs	1.01
utils/has_operators/break_sharing	1.78 ± 0.02 μs	1.91 ± 0.022 μs	0.929
utils/hash/break_sharing	28.9 ± 0.54 μs	30.3 ± 0.58 μs	0.953
utils/index_constants/break_sharing	28.3 ± 0.78 μs	28.2 ± 0.77 μs	1
utils/index_constants/preserve_sharing	0.132 ± 0.0037 ms	0.131 ± 0.0037 ms	1.01
utils/is_constant/break_sharing	4.89 ± 0.23 μs	4.58 ± 0.24 μs	1.07
utils/simplify_tree/break_sharing	0.264 ± 0.021 ms	0.251 ± 0.02 ms	1.05
utils/simplify_tree/preserve_sharing	0.381 ± 0.024 ms	0.41 ± 0.023 ms	0.93
utils/string_tree/break_sharing	0.591 ± 0.017 ms	0.577 ± 0.014 ms	1.02
utils/string_tree/preserve_sharing	0.729 ± 0.023 ms	0.714 ± 0.018 ms	1.02
utils/hash/preserve_sharing		0.136 ± 0.0033 ms
time_to_load	0.707 ± 0.0058 s	0.699 ± 0.014 s	1.01

[coveralls]

Pull Request Test Coverage Report for Build 7336937036

• 29 of 29 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.6%) to 94.254%

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Totals
Change from base Build 7257779121:	0.6%
Covered Lines:	1263
Relevant Lines:	1340

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💛 - Coveralls