Adding Text to Static Analysis Chapter

I've added the beginning (intro sections) of the chapter. I'm going to add 4 more sections, one for each of three "checks" that TypeCheck can make.

There probably needs to be more "this is how this works in Julia" notes added to the early code examples.
General comments on what you've written so far:

- I enjoyed the pacing and structure of your writing. I felt that I was being gently guided through the topic you were explaining, and I was generally very curious at each stage to see what was coming next. You are also very good at explaining the specific concepts in a way that doesn't assume too much about the reader's background.
- I do like the depth of the exploration of Julia's `code_typed` and the data structures it exposes, but I did feel like things were slightly drawn out once we saw the trick of "introspect loops, look for `UnionType`s". I know you had to go into some detail about how Julia represents ASTs, but I wonder if there's not some room for compression there.
- You structured your check as a small 2-stage "pipeline" of functions. [Organizing programs as signal-processing problems](https://mitpress.mit.edu/sicp/full-text/sicp/book/node35.html) may be a useful theme to explore in this chapter? 
- If the exploration and implementation of your next type check is equal or greater in length to this one, I don't think you'll have room for another. 
- As a 'pie in the sky' ideal, what I would really like to see is an discussion of how we would build a staged type-check system that applies multiple checks when invoked. i.e. run a command-line tool on my source tree, and it finds all these things, possibly bailing out if errors make it impossible to continue. You mentioned that Julia has 4 hooks into the reflection system -- would we need to use all of those hooks? If so, would we want each typecheck function to specifically grab at what they need through those functions, or would we want a more consistent way of doing this? How would we stage and register the checks? How would we make it easy for developers to add new checks?

Obviously there wouldn't be much room to add more code to your contribution, but even exploring this concept exclusively in prose would be something I'd be very interested in. Of course, you may not find that nearly so interesting :)