Fix fast-elements

In the last commit I changed it to store in fast-elements with the wrong id.
Unfortunately this can't be picked up in testing, as the fallback still works
correctly. Although it didn't make any notable difference to the build time,
so maybe the fast-elements mechanism isn't needed?

xsl/lookup.xsl

@@ -185,7 +185,7 @@
   <xsl:template match="*" mode="preprocess-ids">
     <element>
       <xsl:attribute name="id">
-        <xsl:call-template name="object.id"/>
+        <xsl:value-of select="generate-id()"/>
       </xsl:attribute>
       <xsl:attribute name="part-id">
         <xsl:call-template name="print-id-part"/>