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.
Couple of things here:

a) I might explicitly make the link for the readers that to implement a static analysis tool for Julia in Julia, the code is going to have to do more than just read the plaintext of the program; it's going to actually need some access to the underlying data structures + intermediate forms of the compiled program, and that's called 'introspection'.

b) It might be useful to save this section for when you get to the 'implementation' step of your 3-step process described in the introduction. After you taught me the absolute basics of Julia in the above section, I was expecting a discussion of the sorts of checks we wanted to do; and then, some high-level discussion of how they can be done, before getting to this kind of detail on the how.