Add `AssociateTo`

[Tiago Cavalcante Trindade]

https://reference.wolfram.com/language/ref/AssociateTo.html

[R. Bernstein]

I started looking at this and it is not too difficult. However it brings up what I think is a weakness we currently have with associations: they aren't stored in any sort of efficient or Python-friendly way, but instead are S-expressions.

And this makes things like AssociateTo a little more cumbersome.

So instead what I am thinking of now is taking a longer view and adding a Python dictionary representation that is most useful here, but also useful in the other operations.

And by its nature suggests adding to_python() and from_python() routines which would also be desirable.

[Tiago Cavalcante Trindade]

This already was added.