More simplification routines

[ChrisRackauckas]

We need more routines to simplify equations down to common forms. It doesn't need to cover all of the fancy cases, but we should be able to have -x be the same as -1 * x.

[smldis]

May want to check
https://github.com/symengine/symengine/blob/master/doc/design.md

[ChrisRackauckas]

Thanks. It's interesting they have so many pointers. I've been wondering about that for Variable, because then Variables as references would be unique instead of copied all around. This would be helpful for making components.

[smldis]

Given julia's similarities with C++ and its current lack of capability of avoiding copies in this case, refs make sense. Is using refs going to also simplify expression manipulation algorithms? (unique means it is easy to check for equality right?)
This could be also handled at a lower level representation, I don't know what the current plans are.

[ChrisRackauckas]

unique means it is easy to check for equality right?

Yeah it would just be references.