combine_fields(+Fields:list, +MsgDict0, -MsgDict) is det[private]

Combines the fields into a dict and sets missing fields to their default values. If the field is marked as 'norepeat' (optional/required), then the last occurrence is kept (as per the protobuf wire spec) If the field is marked as 'repeat', then all the occurrences are put into a list, in order. This code assumes that fields normally occur all together, but can handle (less efficiently) fields not occurring together, as is allowed by the protobuf spec.