Port to protozero

[springmeyer]

We should port the code to protozero because:

• it would make the code easier to build: no protoc to run, no pre-compiled library to link
• it would make the build system easier to maintain and easier to update deps
• it would make the code potentially higher performance

The downside to protozero is that it makes it slightly more work when you want to update the protobuf scheme. However, I think this is a fine tradeoff since the schema here for glyphs has not changed frequently so far.

This in my mind is low urgency, but a nice to have and a good starter task for someone interested in learning and using protozero more.

/cc for visibility @kkaefer @ChrisLoer @mapbox/core-tech