Figure out a better way to load Scheme code into the shared library

[aartaka]

The current approach of literal string insertion is good because

• We don't need to bundle the Scheme file with the library.
• The library doesn't need to reach to an outside file (which is really hard for a WebKitGTK-loaded shared library—it's heavily sandboxed.)

But it's dangerously indirect and opaque. So would be nice to find some other way to integrate Guile, WebKitGTK, and C slightly tighter and in a more correct-ish way.