Represents a single field in an UnknownFieldSet. An UnknownField consists of four lists of values. The lists correspond to the four "wire types" used in the protocol buffer binary format. Normally, only one of the four lists will contain any values, since it is impossible to define a valid message type that declares two different types for the same field number. However, the code is designed to allow for the case where the same unknown field number is encountered using multiple different wire types.