Protobuf Compatibility

Decide whether two .proto sources are compatible, treating the field number as the identity it is. Reports a number freed without a reserved entry, which arms a future field to reinterpret old bytes; a number taken from a reserved range; a wire type change, which stops old messages decoding; a same-wire-type change that alters the decoded value, such as signed to unsigned; a rename, which is wire-safe and breaks generated code; cardinality and oneof moves; and enum number changes.