Use Babel + Rollup for modular builds

[jamesplease]

Rollup allows our users (who are also using Rollup) to pull down just the features of the library that they're using.

[megawac]

Is it ready?

[jamesplease]

It's usable afaict. At worst, it's got the functionality of Esperanto and we're no worse off than any other library. At best, any outstanding bugs with minimizing bundles will be fixed in the coming weeks/months and we save ourselves a lot of work.