Port to protozero

[springmeyer]

This changes the code to use protozero instead of the google libprotobuf library.

More context at https://github.com/mapbox/node-fontnik/issues/143.

I needed this SDF generation for a different project where I did not want to not use the libprotobuf library, so I worked on this first here to ensure the tests were passing.

TODO:

• [ ] Code review
• [ ] Potentially use pbf_builder with strongly typed enums that are clearly named instead of using integers (which make mistakes easier): https://github.com/mapbox/protozero/blob/master/doc/tutorial.md#writing-protobuf-encoded-messages-using-pbf_builder

/cc @mapbox/map-design-team @mapbox/maps-api @mapbox/api-gl

[springmeyer]

Closing, since this work is included in https://github.com/mapbox/node-fontnik/pull/163