dillon1000/react

Add Mermaid visualizations for CFG into fixtures

This PR adds a new section to fixture tests, which renders the HIR into 

a visualization using mermaid.js syntax which can then be embedded 

directly into markdown. 

The nice thing about the mermaid syntax is that it's quite readable, so 

if desired we could replace the current basic block textual output with 

the mermaid block. I'm opting to append it for now and wait for feedback 

if we want to keep both or replace. 

To view the graphs in your editor, download an extension that 

adds mermaid.js support: 
https://mermaid-js.github.io/mermaid/#/integrations?id=editor-plugins. In vscode 
you can use this plugin by right clicking on "Open Preview" on any expect.md 
file. No extra dependencies are required for GitHub which should have builtin 
support for mermaid in markdown