Allow per-variable complexity

[MilesCranmer]

For example, to make x1 have complexity 1, x2 complexity 2, and x3 complexity 3:

Options(
    ...,
    complexity_of_variables=[1, 2, 3],
)

[github-actions]

Benchmark Results

	master	f7a404b...	master/f7a404b127a11d...
search/multithreading	20.8 ± 0.36 s	22.4 ± 0.53 s	0.928
search/serial	33.9 ± 0.19 s	36.3 ± 0.29 s	0.934
utils/best_of_sample	0.731 ± 0.27 μs	0.692 ± 0.3 μs	1.06
utils/check_constraints_x10	11.7 ± 3 μs	11.4 ± 2.8 μs	1.03
utils/compute_complexity_x10/Float64	2.22 ± 0.11 μs	2.11 ± 0.12 μs	1.05
utils/compute_complexity_x10/Int64	2.16 ± 0.11 μs	2.11 ± 0.11 μs	1.02
utils/compute_complexity_x10/nothing	1.47 ± 0.09 μs	1.42 ± 0.1 μs	1.04
utils/optimize_constants_x10	31 ± 7.4 ms	0.0334 ± 0.0072 s	0.926
time_to_load	0.918 ± 0.0036 s	0.922 ± 0.01 s	0.995

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).