riddlr refactor

This project kicked off largely as a proof of concept and focused only on getting a working app out the door while exploring the feasibility of making a "real-world" programming environment available within a shiny app (with code completion, ability to load libraries, syntax checking and available documentation).

I think the feasibility was proven, but the code isn't necessarily approachable. There's a lot of assumed structure being passed around as lists without any constructors.

There are a few things that would tremendously help to keep the code maintainable:

1. `riddle`, `riddle_grader` and `riddle_grader_test` constructors
- `riddle(, )`
- `riddle_grader(, )>)`
- `riddle_grader_test(, , )`
1. Add `parse_riddle_rmd_dir` to build a catalog from a directory of `Rmd` files.
1. Rename `riddle` and `riddle_ui` shiny module constructors to `shinyRiddle` and `shinyRiddleUI` and migrate to using riddle class as input instead of independent fields.

The issue names no files or tests. Start by tracing the existing riddle and riddle_ui constructors and their list-shaped inputs, then map the requested constructors, Rmd-directory parser, and Shiny module rename. Done means the listed constructors and parser exist, and the Shiny modules accept the riddle class input as specified.

Written by the indexing model from the issue text.