Return promises and/or accept callbacks for all async methods

Nobody has claimed this yet.

Most of the maps I'm working on at the moment have the basic pattern:

- load a basemap from Mapbox (either a standard one, or slightly tweaked)
- add stuff to it with Mapbox-GL-JS, such as a GeoJSON specified by URL

But you can't add stuff to the map until it's fully loaded. So you either:

1: Wait for the map to load, then add the source by URL. This is slow, and misses out on the opportunity to do the two network requests in parallel.

2. Fetch the GeoJSON yourself (in parallel), and then add it when the map is ready. You probably end up adding D3-request or similar. Then you end up with this annoying code:

```
if (map.loaded())
map.addSource(...);
else
map.on('load', () => map.addSource(...));
```

Is there an inherent reason `map` can't accept new sources while it's loading?

Start by tracing the map load lifecycle and the async methods discussed in the issue; no files or tests are named. Compare the requested promise or callback behavior with the current source-loading flow, and consider the work done when the agreed API supports the affected async methods during loading.

Written by the indexing model from the issue text.