Remove old implementation of read and code gen phases

[chrispcampbell]

In #445 we made the new implementation of the parse/read/gen phases the default, but left the old implementation in place (accessible with an environment variable) for the time being in case anyone needs time to transition. Once the new implementation has been the default for a few weeks, we can remove the old implementation files.

• Remove the old files (see Add new implementation of read and code gen phases based on parse package #412 for a list of old files and their new replacements)
• Update the Transpiler Architecture wiki page to correctly reference new files and functions (instead of the old classes)

[chrispcampbell]

As noted in the checklist above, I updated the Transpiler Architecture wiki page to describe the new structure and functions. For comparison, the old version can be found here and the diff between old and new is here.

The new page uses Mermaid for the flow charts and diagrams so that we don't need to maintain images for these.

Also worth noting that the page is written as if the JS target is complete (it says C and JS files can be generated), which won't be true until the JS target is ready, but I'm hoping that will be ready in the near future and I think it's more illustrative to show it this way.