Add a sideEffects: false flag for Webpack 4

[timdorr]

See https://github.com/webpack/webpack/tree/master/examples/side-effects

Webpack 4 can more effectively tree shake when a module defines itself as having no side effects (aka, doesn't talk to things external to the module at the top level). I believe we qualify, but I'm putting this in as a PR just to be sure. Looks like it can easily net some byte savings for end users 👍

[nickserv]

What's involved with testing this, would it be enough to run CRA on this build of Redux and ensure that it doesn't break? Or would the module have to be analyzed?

[timdorr]

Build it with webpack 4 and see if it breaks. I think there's a low likelihood of problems, so I'll throw it into the next release (probably an RC) and see if everyone throws tomatoes back.