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.
This paragraph might better fit in the implementation section. You could open the section by asking "OK, so how are we going to detect that this is even happening in the first place without actually running the code? It turns out that Julia has a special type for variables that can hold values of multiple different types..."