Consider putting context inside Asts

[zenna]

Write now all my functions take a context as a kwarg. This is cumbersome, and prone to the error of combining things which do not have the same context.

Also it's not good if there is some external function, written without the context passing required.

One solution would be to enclose a reference to the context in any variable. Then only allow operations of variables in the same context. Then we wouldnt need to keep passing contexts around to all functions.