Investigate enabling Promise support in Cell

[fatcerberus]

Attempting to use async functions or promises in Cell will currently throw an error like this:

Error: No async/promise continuation support
   at Module code ($/Cellscript.js:15:1)

Promise continuations are deliberately disabled because Cell doesn't have the necessary event loop hooks to support them. It's not even entirely clear where these hooks should be, whether they'd be processed between targets, or something else. It's also not clear what to do if a tool in the middle of building a target awaits an asynchronous operation: should Cell attempt to move on to the next (top-level) target? What if the only remaining targets to build are dependant on the one stuck in the middle of the await? Then what?

Unlike miniSphere which can guarantee several ticks (in the form of frames) per second, Cell doesn't have this luxury and introducing an event loop is going to require some serious thought to make sure things don't go wrong. Tentatively scheduling this for miniSphere 6.0, but time will tell if this is too ambitious.