Use (shallow) merge semantics for `map.setStyle()`

Nobody has claimed this yet.

## Motivation

`Map#setStyle()` currently does a deep equality comparison when diffing sources: https://github.com/mapbox/mapbox-gl-js/blob/a5a8dfe5b90737dbb295eaca7f41c667ae4060a8/src/style-spec/diff.js#L153

This is unnecessarily costly for styles using inline / dynamically-created GeoJSON data. And more generally, it's very common that the caller of `map.setStyle()` knows that the sources haven't changed.

See also @averas's comment here: https://github.com/mapbox/mapbox-gl-js/pull/3643#issuecomment-275741172

## Design Alternatives

#4006
#4000

## Design

Per the title, I propose that we do a shallow merge from `map.setStyle()`. In practice, this means that a user could omit omit any top-level field (`sources`, `layers`, etc.) of the style they're passing to `map.setStyle()`, and we would just use the existing style state for that field.

### Mock-Up

```js
map.setStyle({
layers: [ ... new layers ...]
// don't worry about setting sources:, sprite:, etc.
})
```

Start at map.setStyle() and inspect the source diffing logic in src/style-spec/diff.js at the linked comparison. Trace how omitted top-level style fields are handled and how source equality is checked; done means omitted fields preserve existing style state and source comparison uses shallow merge semantics rather than deep equality.

Written by the indexing model from the issue text.