Clean up WireReference and add a sign bit to offset.

c++/src/capnproto/wire-format-test.c++

@@ -40,7 +40,7 @@ namespace {
 TEST(WireFormat, SimpleRawDataStruct) {
   AlignedData<2> data = {{
     // Struct ref, offset = 1, fieldCount = 1, dataSize = 1, referenceCount = 0
-    0x08, 0x00, 0x00, 0x00, 0x01, 0x01, 0x00, 0x00,
+    0x04, 0x00, 0x00, 0x00, 0x01, 0x01, 0x00, 0x00,
     // Content for the data segment.
     0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef
   }};