Use rollup instead of webpack for small modules

[lucat1]

This will lower and speed up the build process. And luckily fix the errors that we're getting in the CI builds. Plus rollup provides and amazing es6 configuration and a really tiny footprint, with blazing-fast performance. We'd also replace it in the demo if it supports chunk splitting! 🎉

This implementation would actually lower the size BY A LOT, especially when using polished:

What is "tree-shaking?"

Tree-shaking, also known as "live code inclusion," is the process of eliminating code that is not actually used in a given project. It is similar to dead code elimination but can be much more efficient.

The major reason for this is shrinking the size even further

[lucat1]

Handling in #48