we need more integral solvers

There are a lot of stuff to do, but to begin with, I will put some unsolvable by AM integrals here:
- [ ] `cos(x^2)*x` variable substitution method, [WolframAlpha](https://www.wolframalpha.com/input/?i=integrate+cos%28x%5E2%29*x)
- [ ] `sin(x)*e^x` integrating by parts and solving integral equation, [WolframAlpha](https://www.wolframalpha.com/input/?i=integrate+sin%28x%29*e%5Ex)
- [ ] `1/(a^2 + x^2)` standard integral for arctan, [WolframAlpha](https://www.wolframalpha.com/input/?i=integrate+1%2F%28a%5E2+%2B+x%5E2%29)
- [ ] `x^2 / (x^4 + 1)` partial fractioning [WolframAlpha](https://www.wolframalpha.com/input/?i=integrate+x%5E2+%2F+%28x%5E4+%2B+1%29)
- [ ] `sqrt(tan(x))` very painful, requires different solvers, [WolframAlpha](https://www.wolframalpha.com/input/?i=integrate+sqrt%28tan%28x%29%29)

I **do not** mention special integrals here, like `e^(x^2)` or `1/ln(x)`