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.
By the time the book is ready to go, we will also have a couple of chapters that deal with producing and consuming ASTs, so we can point readers there also.