Port Subscription closure implementation from 8.x to 7.x

[markerikson]

In #1755, I figured out that we could rewrite the internal Subscription class as a closure instead. This shaves off some bytes thanks to closures minifying better.

However, that change went straight into our master branch as part of v8.x development. We should be able to easily backport this to the 7.x branch and release it in the next patch update.

I'd love to see someone file a PR that makes those same changes to 7.x.