Speed up binary decoding by up to 2x
Speed up binary decoding by up to 2x

This change simply defers the unknown field tracking
until we know the field is truly unknown.

allevato/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,allevato/swift-protobuf,apple/swift-protobuf,tbkka/swift-protobuf,cburrows/swift-protobuf,thomasvl/swift-protobuf,tbkka/swift-protobuf,tbkka/swift-protobuf,cburrows/swift-protobuf,apple/swift-protobuf,cburrows/swift-protobuf,thomasvl/swift-protobuf,tbkka/swift-protobuf,cburrows/swift-protobuf,tbkka/swift-protobuf,allevato/swift-protobuf,cburrows/swift-protobuf,thomasvl/swift-protobuf,cburrows/swift-protobuf,thomasvl/swift-protobuf,allevato/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,cburrows/swift-protobuf,tbkka/swift-protobuf,apple/swift-protobuf,allevato/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,tbkka/swift-protobuf,thomasvl/swift-protobuf,allevato/swift-protobuf,allevato/swift-protobuf