Create cond operator

[MilesCranmer]

This seems like a fairly common operator for people to make by themselves, so might as well add it as a default option.

cond(x, y) = (x > zero(x)) * y

Basically an if statement with the false branch set to 0.

This also redefines a few of the logical operators to be SIMD-compatible by using multiplication-based definitions.

TODO:

• Ensure operators are SIMD compatible in unittest

[github-actions]

Benchmark Results

	master	f555298...	t[master]/t[f555298...]
search/multithreading	27.7 ± 4.2 s	26.9 ± 3.7 s	1.03
search/serial	34 ± 0.94 s	34.1 ± 0.91 s	0.999
time_to_load	2.03 ± 0.012 s	2.03 ± 0.0064 s	1
utils/best_of_sample	0.952 ± 0.39 μs	0.892 ± 0.41 μs	1.07
utils/check_constraints_x10	12.6 ± 3.2 μs	12.7 ± 3.2 μs	0.995
utils/compute_complexity_x10/Float64	2.37 ± 0.13 μs	2.31 ± 0.12 μs	1.03
utils/compute_complexity_x10/Int64	2.32 ± 0.12 μs	2.31 ± 0.11 μs	1.01
utils/compute_complexity_x10/nothing	1.58 ± 0.12 μs	1.5 ± 0.12 μs	1.05
utils/optimize_constants_x10	30.8 ± 6.9 ms	31.5 ± 6.4 ms	0.978

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).