Refactor parts of codebase

[MilesCranmer]

This modularizes parts of the codebase to improve readability. I've closely checked it's identical in function but worth triple-checking.

[github-actions]

Benchmark Results

	master	34ee657...	master / 34ee657...
search/multithreading	17.3 ± 0.58 s	17.5 ± 0.48 s	0.992
search/serial	29.9 ± 0.14 s	30.3 ± 0.46 s	0.988
utils/best_of_sample	1.61 ± 0.61 μs	2 ± 1.2 μs	0.805
utils/check_constraints_x10	11.2 ± 2.9 μs	11.5 ± 2.9 μs	0.979
utils/compute_complexity_x10/Float64	2.14 ± 0.2 μs	2.15 ± 0.23 μs	0.995
utils/compute_complexity_x10/Int64	2.13 ± 0.23 μs	2.11 ± 0.23 μs	1.01
utils/compute_complexity_x10/nothing	1.46 ± 0.2 μs	1.53 ± 0.6 μs	0.954
utils/insert_random_op_x10	5.99 ± 2.1 μs	5.89 ± 2.2 μs	1.02
utils/next_generation_x100	0.405 ± 0.056 ms	0.411 ± 0.093 ms	0.984
utils/optimize_constants_x10	0.037 ± 0.0083 s	0.0369 ± 0.0082 s	1
utils/randomly_rotate_tree_x10	5.44 ± 0.67 μs	5.3 ± 0.63 μs	1.03
time_to_load	1.92 ± 0.054 s	1.97 ± 0.026 s	0.974

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 13955420704

Details

• 173 of 176 (98.3%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 95.439%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ProgressBars.jl	8	9	88.89%
src/SymbolicRegression.jl	100	102	98.04%

Totals
Change from base Build 13607350038:	0.02%
Covered Lines:	3390
Relevant Lines:	3552

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💛 - Coveralls