In the initial implementation, we had a config option to determine
whether unknown fields should return an error.  Based on the discussion
in [0], I think it is better to just collect unexpected fields in the
message type and let the user decide how to handle them.

[0] stadelmanma#13

The name for unknown fields is just unknown_field_n, so it does not
provide any additional value.  Using a map instead of a vec makes it
easier to access the data.