Move non-core built-ins to optional libraries

[jathak]

While it's not as bad as the old interpreter in this regard, there's a decent chunk of built-in procedures that aren't really relevant to this interpreter's core purpose: teaching 61A students Scheme.

To that end, I'd like to restrict the core library to the following:

• Core functionality implemented in the project and described in the 61A Scheme specification
• Core learning and teaching tools that course staff show students (or use themselves)
  • Mainly the diagramming and documentation procedures
• Core quality-of-life utilities that are frequently used
  • clear, bindings, theme, maybe a few others

For everything else, we'll move them into a set of core libraries that can be imported when needed, similar to how the Logic library currently works (though we'll only have a single library procedure for imports, rather than a separate loading procedure for each library).

tldr: Does it spark joy teach Scheme? If not, throw it in an optional library.