Implement OriginalMapping as an autovalue rather than protobuf message

[niloc132]

Enables downstream projects to depend only on closure-compiler (and not protobuf-java) to read and write sourcemaps.

The proto file has shrunk over time, and only has a single message type, used to communicate details discovered when reading a sourcemap from JSON, composed into a message structure, and then back into separate values. The proto-based implementation doesn't serialize or deserialize, and doesn't appear to be involved in CompilerState even indirectly.

The replacement is slightly stricter - rather than marking each field as optional, only the identifier is optional.