Switch from protobuf crate to prost

[aljoscha]

The protobuf crate doesn't make for very idiomatic rust code. Prost is nicer but doesn't support reflection/Any. Alternatively, we could try and support switching between Protobuf implementations.

These are the relevant Prost issues:

• https://github.com/danburkert/prost/issues/235
• https://github.com/danburkert/prost/issues/301
• https://github.com/danburkert/prost/issues/277
• https://github.com/danburkert/prost/issues/299

Relevant PR:

• https://github.com/danburkert/prost/pull/336