Make async things a bit nicer

[matthewturk]

This switches to using the import operation to bring in the webassembly. We can delay some things, like initialization of listeners (so no operations can happen on colormaps until that happens) but it looks like the render operation for fixed res still needs a promise.

Alternately, a considerably safer thing to do might be to wrap all our functions in promises that may be immediately resolved; for instance:

someFunction: function(){ return yt_tools.then( (yt_tools) => {
...
})})

[matthewturk]

I've changed this to use the "safer" version I note above.

[munkm]

I also think this addresses the major issues we were having with #9 .