Refactor Map, introducing Style and Camera as public APIs

[jfirebaugh]

In the current Mapbox GL JS API, almost the entirety of the API surface area lives on Map, making it a God object.

In particular, it isn't possible to manipulate a style via the runtime styling API, or a camera via the camera API, without either being associated with a live-rendering map. Separating Style and Camera classes from Map would improve Mapbox GL JS's internal architecture and make the external API more flexible. For example, a map-independent Camera would be a natural way to implement Map-independent projection/unprojection. In addition, it would bring GL JS closer to Native, which has public Style and Layer classes (though not Camera, yet).

This is a continuation/reframing of #2741.