Introduce Closure Binding meaningfully for event-handling lesson

[tildeee]

I really want to bring up closure binding again. I think it's ok to skip it here, since if we squint at this, we can convince ourselves that we're just looking at a funny looking class. But eventually closure semantics wind up rearing their head and ordering can become an issue (especially with useCallback dependencies).

Originally posted by @anselrognlie in #38 (comment)