def write_pair(field, value)
      key = (field.tag << 3) | field.wire_type
      stream << ::Protobuf::Field::VarintField.encode(key)
      stream << field.encode(value)
    end
def write_pair(field, value)
      key = (field.tag << 3) | field.wire_type
      stream << ::Protobuf::Field::VarintField.encode(key)
      stream << field.encode(value)
    end
Encode key and value, and write to +stream+.