Guide cargo thru functions

My clarifications around variables and bindings may be controversial (although I think they're helpful), so be sure to take a look.

These edits include changes from sections 3 through 6 (Hello, Cargo to the end of Functions).

@steveklabnik 
I don't like this change either, it sounds more complex, if that makes any sense. "initializer" sounds like some kind of jargon.