Replace function expressions with arrow expressions

[k4b7]

I was experimenting and it does cut down on the bundle size. It looks like terser-js can do this for anonymous function expressions. My guess is that it won't do it for named ones. We could add this for all functions that start with a lowercase letter b/c those are known to not be constructors and thus don't need access to this. Would you be interested in a PR for this?