merge icon and text render types into symbol type

Text and icon render types are merged into the symbol render type. With the types combined, we can place points + text together which means better placement and support for shields. #67.

A layer with the symbol render type can specify both text and icon properties. Some properties, those prefixed with symbol-, affect both.

- `symbol-placement` controls whether placement happens at a point or along a line
- `symbol-min-distance` replaces `icon-spacing` and `text-min-distance`
- `symbol-allow-overlap` and `symbol-ignore-placement` replace the text- and icon- versions
- `text-upright` and `icon-upright` control whether the symbols can get flipped
- `symbol-rotation-alignment` controls whether the symbol rotates with the map, replacing `text-path` and `icon-rotate-anchor`

If this looks ok I'll update the conversion script.

@mourner @kkaefer @yhahn