[WIP] Reify cowns and add taint for visual story telling

[xFrednet]

This PR reifies cowns. These are objects, which are opaque by default, meaning that the linked region can't be accessed unless they're acquired. Currently, there is no mechanism to acquire a cown, but I already added checks for it in the required places.

Mermaid is aware of cowns and draws the node with two lines on the side and dotted outgoing edges. This is meant to show that they're not directly accessible. I'm open to any suggestions how to make this clearer or what to try out.

The taint instruction colors everything purple that can be reached from a given set of root nodes. For now only one root can be defined at a time. Cowns stop the propagation of taint.

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Here is a taint example from valid_cowns.vpy

There is a taint example from three_regions.vpy, when the regions are still separate

There is a taint example from three_regions.vpy, after they have been connected

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cc: @TobiasWrigstad in case you have some ideas how the visualization could be improved

[xFrednet]

I would also like to support a hide command, which can be used to hide nodes from mermaid. I think it would make sense to implement builtin functions first. Then we can make such additions, without the need to touch the parser anytime.

For this PR, I'll leave taint as a keyword, but I'll consider making it a buildin function instead soon.

[xFrednet]

Thinking about it a bit more, I think that built-in functions would make this whole PR a lot better. I could remove all the parser changes and have the cown creation done with a built-in function as well

[xFrednet]

I'll reimplement this on top of my changes in the 16-build-out-buildin-funcs branch. That should be cleaner and more flexible.