Implement Mermaid diagrams

[glesica]

See https://mermaid-js.github.io/mermaid/#/./classDiagram

There are still some issues:

1. There is no name spacing, which doesn't seem to be supported by Mermaid

Since name spacing doesn't appear to be supported, just assume it's not an issue. We can revisit later, but there's really no good solution other than mashing the names together.

2. Only limited testing so far (just using DCDG itself)

Added some tests. Beyond that, existing tests are fairly general. Would still like to add a gold file test for each builder.

3. Type names with () cause fields to show up as methods

Just elide the parens and call it a day. For most use cases this should be fine. We may need to think about callbacks with parameters.