Drop `npm-utils` and provide pre-bundled runtime

Following #338, we should be able to avoid running parcel at link-time and instead ship a pre-bundled rts.js. Some minor adjustments in the js codegen, and the browser backend can take advantage of rts.js and use it to initialize multiple instances at once.

Besides improving linking speed, this also means we can drop npm-utils, which uses a custom Setup.hs to do npm install...which is definitely not a good thing.