bolt-gen: fixup the devtool/decodemsg printing facility

Fixup TLV handling in the bolt printing utility, `devtools/decodemsg`

tools/generate-bolts.py

@@ -270,6 +270,9 @@ class Type(FieldSet):
     def is_subtype(self):
         return bool(self.fields)
 
+    def is_truncated(self):
+        return self.name in self.truncated_typedefs
+
     def needs_context(self):
         return self.is_varsize() or any([field.needs_context() for field in self.fields.values()])
 
@@ -329,6 +332,9 @@ class Tlv(object):
                                            struct_prefix=self.struct_name(),
                                            comments=comments)
 
+    def type_name(self):
+        return 'struct ' + self.struct_name()
+
     def struct_name(self):
         return "tlv_{}".format(self.name)