Use array of structs rather than parallel arrays for annotations

[kkaefer]

Our current point annotations API uses two parallel vectors for points and icon names which leaves room for errors due to mismatched annotations. Instead, we should use an array of structs so the association between point and icon is fixed and not through the index.

[1ec5]

👍

[1ec5]

@kkaefer, per chat with @incanus, I reverted this change in 714b68f, because it complicated landing #1655. I like the idea of making annotations more object-oriented at the mbgl level, but that’s a lower priority than landing shape annotations.