support substraction on Tangent{T}

[oxinabox]

We could support subtraction on Tangent{T} just as -(t1,t2) = t1 + -1*t2

Diffractor has an internal higher order rule that assumed such things exist. (https://github.com/JuliaDiff/Diffractor.jl/blob/cbcc0f3ffcedd8074c91a261b7e19229dcb273b7/src/higher_fwd_rules.jl#L44)
But I am not sure if people really should expect to be able to do this for structural tangents.
(I am currently working with a weird number type kinda like a unitful thing but less comprehensive)

[oxinabox]

If implementing this we should do it for all tangent types.
This need was mentioned in #639
as well.
And I think it makes sense for writing gradient descent etc.

@lukas-weber would you like to take a crack at this?

[lukas-weber]

Sure can do!