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 feel like this paragraph is going to be difficult for many to grok. e.g. for ctype-ish programmers, the first analogy that comes to mind is 'primitive types', and they might think "when I do `x += 4` in Java, I'm changing the value, aren't I?" i.e. their minds will first go to reference types vs primitive types. Perhaps a direct comparison to that concept could help? You've already done a similar comparison to Java and C++ when explaining multiple dispatch.)