Mermaid: Small adjustments

[xFrednet]

The list of current changes:

• Increased font size to 16px
• Rename nullptr node to None
• Move region information into the bride object
• Indicate borrows with dotted arrows
• Don't draw the immutable region by default
• Remove info node from Mermaid
• Show nested regions as subgraphs
• Add more asserts to cown creation, now that bridge objects have been reified

Example:

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Closes #35

[xFrednet]

This PR should be ready, besides the CI timeout.

I enjoy working on these diagrams :D