[WIP] feat: add Const variant

[AayushSabharwal]

Needs to merge to #754, will retarget once that merges into the v4 branch on this repo.

[github-actions]

Benchmark Results

	master	6b828ab...	master / 6b828ab...
overhead/acrule/a+2	0.988 ± 0.29 μs	0.2 ± 0.087 μs	4.94 ± 2.6
overhead/acrule/a+2+b	0.985 ± 0.24 μs	0.199 ± 0.088 μs	4.95 ± 2.5
overhead/acrule/a+b	0.267 ± 0.032 μs	0.2 ± 0.088 μs	1.34 ± 0.61
overhead/acrule/noop:Int	1.55 ± 0.01 ns	1.55 ± 0.01 ns	0.999 ± 0.0091
overhead/acrule/noop:Sym	27 ± 6.2 ns	21.3 ± 0.25 ns	1.27 ± 0.29
overhead/get_degrees/large_poly	0.148 ± 0.0066 s	0.115 ± 0.011 s	1.29 ± 0.14
overhead/rule/noop:Int	0.0354 ± 0.024 μs	0.0535 ± 0.025 μs	0.661 ± 0.54
overhead/rule/noop:Sym	0.033 ± 0.023 μs	0.0517 ± 0.025 μs	0.639 ± 0.55
overhead/rule/noop:Term	0.033 ± 0.024 μs	0.0513 ± 0.025 μs	0.644 ± 0.55
overhead/ruleset/noop:Int	0.12 ± 0.013 μs	0.0556 ± 0.039 μs	2.15 ± 1.5
overhead/ruleset/noop:Sym	0.127 ± 0.038 μs	0.0541 ± 0.042 μs	2.34 ± 1.9
overhead/ruleset/noop:Term	4.87 ± 0.76 μs	3.6 ± 0.079 μs	1.35 ± 0.21
overhead/simplify/noop:Int	0.278 ± 0.029 μs	1.55 ± 0.01 ns	179 ± 18
overhead/simplify/noop:Sym	0.269 ± 0.034 μs	0.0602 ± 0.00079 μs	4.47 ± 0.58
overhead/simplify/noop:Term	0.0502 ± 0.0056 ms	13.9 ± 0.47 μs	3.6 ± 0.42
overhead/simplify/randterm (+, *):serial	0.125 ± 0.0098 s	0.042 ± 0.0037 s	2.98 ± 0.35
overhead/simplify/randterm (+, *):thread	0.0714 ± 0.0053 s	0.0325 ± 0.0062 s	2.19 ± 0.45
overhead/simplify/randterm (/, *):serial	0.265 ± 0.026 ms	0.71 ± 0.11 ms	0.374 ± 0.07
overhead/simplify/randterm (/, *):thread	0.298 ± 0.029 ms	0.753 ± 0.031 ms	0.396 ± 0.042
overhead/substitute/a	0.107 ± 0.01 ms	0.0651 ± 0.0037 ms	1.64 ± 0.18
overhead/substitute/a,b	0.0923 ± 0.01 ms	0.0594 ± 0.0036 ms	1.55 ± 0.2
overhead/substitute/a,b,c	21.2 ± 1.8 μs	21.4 ± 2 μs	0.992 ± 0.13
polyform/easy_iszero	0.0417 ± 0.0066 ms	0.039 ± 0.00094 ms	1.07 ± 0.17
polyform/isone	2.79 ± 0.01 ns	7.73 ± 0.011 ns	0.361 ± 0.0014
polyform/iszero	1.41 ± 0.029 ms	1.82 ± 0.15 ms	0.775 ± 0.064
polyform/simplify_fractions	1.93 ± 0.061 ms	2.27 ± 0.17 ms	0.85 ± 0.069
time_to_load	1.12 ± 0.0027 s	1.18 ± 0.016 s	0.946 ± 0.013

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).