Consider using brocolli + rollup

[jrhorn424]

This would give us (1) a huge performance increase on builds and livereload, and (2) allow us to use a simpler sourcemapping scheme (we have to use an expensive one due to the infamous chrome bug #17).

Technical details here, but it boils down to how webpack versus rollup manages isolated scope:

• https://nolanlawson.com/2016/08/15/the-cost-of-small-modules/
• https://www.npmjs.com/package/broccoli-rollup