Add more trig identities

[derikk]

Now we can have simplify(1 - sin(x)^2) == cos(x)^2 instead of just simplify(sin(x)^2 - 1) == -(cos(x)^2).

Also fixed @acrule(tan(~x)^2 + -1*sec(~x)^2 => one(~x)) by adding a minus sign.

[github-actions]

Benchmark Results

	master	940469b...	t[master]/t[940469b...]
overhead/acrule/a+2	1.2 ± 0.29 μs	1.2 ± 0.032 μs	1
overhead/acrule/a+2+b	1.19 ± 0.3 μs	1.18 ± 0.025 μs	1.01
overhead/acrule/a+b	0.433 ± 0.0094 μs	0.426 ± 0.013 μs	1.02
overhead/acrule/noop:Int	0.0317 ± 0.0008 μs	0.0321 ± 0.0007 μs	0.988
overhead/acrule/noop:Sym	0.0614 ± 0.01 μs	0.0553 ± 0.0087 μs	1.11
overhead/rule/noop:Int	0.055 ± 0.0021 μs	0.0534 ± 0.0027 μs	1.03
overhead/rule/noop:Sym	0.0684 ± 0.003 μs	0.0691 ± 0.0025 μs	0.991
overhead/rule/noop:Term	0.0686 ± 0.0046 μs	0.0685 ± 0.0019 μs	1
overhead/ruleset/noop:Int	0.199 ± 0.0026 μs	0.2 ± 0.0047 μs	0.998
overhead/ruleset/noop:Sym	0.225 ± 0.0066 μs	0.227 ± 0.0059 μs	0.992
overhead/ruleset/noop:Term	4.99 ± 0.29 μs	5.04 ± 0.3 μs	0.99
overhead/simplify/noop:Int	0.265 ± 0.0089 μs	0.277 ± 0.014 μs	0.957
overhead/simplify/noop:Sym	0.289 ± 0.0041 μs	0.288 ± 0.0045 μs	1
overhead/simplify/noop:Term	0.0506 ± 0.0046 ms	0.0527 ± 0.0047 ms	0.96
overhead/simplify/randterm (+, *):serial	0.182 ± 0.0017 s	0.184 ± 0.0014 s	0.988
overhead/simplify/randterm (+, *):thread	0.126 ± 0.017 s	0.126 ± 0.015 s	0.999
overhead/simplify/randterm (/, *):serial	0.32 ± 0.01 ms	0.326 ± 0.011 ms	0.982
overhead/simplify/randterm (/, *):thread	0.364 ± 0.012 ms	0.371 ± 0.012 ms	0.981
overhead/substitute/a	0.0837 ± 0.0018 ms	0.0828 ± 0.002 ms	1.01
overhead/substitute/a,b	0.0746 ± 0.0048 ms	0.0721 ± 0.0017 ms	1.03
overhead/substitute/a,b,c	25.6 ± 0.9 μs	25.2 ± 0.6 μs	1.02
polyform/easy_iszero	0.044 ± 0.0039 ms	0.0442 ± 0.0043 ms	0.995
polyform/isone	3.7 ± 0.1 ns	3.6 ± 0.1 ns	1.03
polyform/iszero	2.75 ± 0.04 ms	2.76 ± 0.038 ms	0.997
polyform/simplify_fractions	3.73 ± 0.042 ms	3.72 ± 0.045 ms	1
time_to_load	6.27 ± 0.055 s	6.23 ± 0.039 s	1.01

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).

[oameye]

why is this kept from merging?