Make U+2016 and U+2980 symmetric

This make things consistent with other stretchy fences.

w3c/mathml#176

Author: fred-wang

unicode.xml

@@ -36658,8 +36658,8 @@ Barbara Beeton for the STIX project
             <desc>alias ISOTECH Verbar</desc>
          </entity>
          <font name="hlcry" pos="107"/>
-         <operator-dictionary priority="20" form="prefix" fence="true" stretchy="true" lspace="0" rspace="0"/>
-         <operator-dictionary priority="20" form="postfix" fence="true" stretchy="true" lspace="0" rspace="0"/>
+         <operator-dictionary priority="20" form="prefix" fence="true" stretchy="true" symmetric="true" lspace="0" rspace="0"/>
+         <operator-dictionary priority="20" form="postfix" fence="true" stretchy="true" symmetric="true" lspace="0" rspace="0"/>
          <description unicode="1.1">DOUBLE VERTICAL LINE</description>
       </character>
       <character id="U02017" dec="8215" type="other" mode="unknown">
@@ -59040,8 +59040,8 @@ Barbara Beeton for the STIX project
          </Elsevier>
          <AMS>\Vvert</AMS>
          <entity id="tverbar" set="STIX"/>
-         <operator-dictionary priority="20" form="prefix" fence="true" stretchy="true" lspace="0" rspace="0"/>
-         <operator-dictionary priority="20" form="postfix" fence="true" stretchy="true" lspace="0" rspace="0"/>
+         <operator-dictionary priority="20" form="prefix" fence="true" stretchy="true" symmetric="true" lspace="0" rspace="0"/>
+         <operator-dictionary priority="20" form="postfix" fence="true" stretchy="true" symmetric="true" lspace="0" rspace="0"/>
          <description unicode="3.2">TRIPLE VERTICAL BAR DELIMITER</description>
       </character>
       <character id="U02981" dec="10625" type="other" mode="unknown">