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.
Yes, Julia uses "function" for a generic function. The function has one or more "method"s, which each have a specific type signature. You have a function of a given name; the function exists as soon as it has at least one method. Each type your write `function foo(...)`, you are implementing a method of `foo`.