Add support for rendering Cryptol specs to HTML and possibly other formats

[yav]

It might be nice to have a tool, similar to rustdoc, javadoc, haddock, or doxygen, for generating HTML (and possibly other formats) from Cryptol specifications. The basics of this should be fairly straightforward with all the infrastructure we have.
Some features we might want to support:

• API view: show public functions with their types and documentation
• Source view: view the definition of a function
• Syntax highlighting: this can be useful for both the API and source view
• Hyperlinked source: navigate between symbols by clicking on them
• Indexing and searching