search engine: first draft of code and also (slightly desynchronized) chapter text!

That which files are all in the same directory? The files we're indexing? (And then I guess store the index in a non-flat fashion in a fixed place inside of it, like `./.chispa`?) But the code still has to worry about which directory that is in order to find the files for e.g. `grep`, and I'm not sure that worrying will be noticeably less complicated. The only simplification would be that `find_documents` would be like three lines instead of four, no?

It also seems like it would make the engine a lot less useful.