java_gen: renamed OFChecksum128 to U128 + implement HashValue

This has the same semantics as our other Uxx types.
diff --git a/test_data/of13/bsn_gentable_entry_desc_stats_reply.data b/test_data/of13/bsn_gentable_entry_desc_stats_reply.data
index 4035f4c..21291ed 100644
--- a/test_data/of13/bsn_gentable_entry_desc_stats_reply.data
+++ b/test_data/of13/bsn_gentable_entry_desc_stats_reply.data
@@ -55,7 +55,7 @@
     .setEntries(
         ImmutableList.<OFBsnGentableEntryDescStatsEntry>of(
             factory.buildBsnGentableEntryDescStatsEntry()
-                .setChecksum(OFChecksum128.of(0xFEDCBA9876543210L, 0xFFEECCBBAA998800L))
+                .setChecksum(U128.of(0xFEDCBA9876543210L, 0xFFEECCBBAA998800L))
                 .setKey(ImmutableList.<OFBsnTlv>of(
                     factory.bsnTlvs().port(OFPort.of(5))
                 ))
@@ -64,7 +64,7 @@
                 ))
                 .build(),
             factory.buildBsnGentableEntryDescStatsEntry()
-                .setChecksum(OFChecksum128.of(0xFEDCBA9876543210L, 0xFFEECCBBAA998801L))
+                .setChecksum(U128.of(0xFEDCBA9876543210L, 0xFFEECCBBAA998801L))
                 .setKey(ImmutableList.<OFBsnTlv>of(
                     factory.bsnTlvs().port(OFPort.of(6))
                 ))