Switch to bs.obj

[mlms13]

We probably can't immediately take advantage of Bucklescript v7's records-as-js-objects since so many of the Mapbox objects expect fields to be missing (instead of undefined values), but we could consider switching from [@bs.deriving abstract] to a bs.obj implementation.

This has the advantage of being slightly more straightforward to read than the magical derived functions that come from @bs.deriving abstract, plus for simply constructing objects, we don't actually need any of the generated getter functions.