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.
I think this part might require more explanation:
> but `Expr` is a type that models the structure used in the Lisp parser.

It seems a little out of context and might be lost on the audience. Maybe just a brief sentence or two on what the structure of the Lisp parser is (or drop the reference and explain the `Expr` type without it? Not sure.)