Value inspection entry point

Conversation with @peterbecich led to this idea, which is a stepping stone toward #741.

Define a type class that captures the notion of being able to inspect a type in a GUI. Define instances for ALL types in the `codeworld-base` package, even if that means that the notion of "inspect" for some atomic values is useless. Also make it easy to define an instance via Generic. Implement a GHC source plugin that auto-derives both Generic and this new type class (via Generic) for all types defined in user code.

Now, add a CodeWorld entry point that uses this type class to display a GUI value inspector, for the value passed in.

Optional add-on: in another GHC source plugin, if `program` is not defined in a codeworld-mode source file, auto-define it to pop up the value inspector for all top-level values defined in the module. Now it's possible to run a program without defining program at all, and if you do, you just get to poke at the values of everything you've defined in the module.