factor progressiveFetch into tools.js for reuse by other pages

[ickk]

I see your point about iterative development being a pain if you needed to rerun the generate_wasm_examples.sh script every time. I still think that long-term we should try to move in that direction, when we have better tooling or when the design is more settled; needing to pass in the function to init really sucks, but I agree it is probably the best option we have right now.

This is a compromise; I think keeping progressiveFetch factored out makes it tidier (hides the polyfill junk), easier to read/reason about, and will allow us to use similar functionality else-where without duplicating code too heavily (This will be good for blog posts/release notes/et c.).

other small changes:

• had to remove some of your comments in the generate script because they broke the escaped Returns.
• shortened comments in general. If someone wants to know what's going on, one link to your PR should be enough to get them up to speed.
• put template's comment inside Tera/Zola comment syntax so it won't be rendered