c:sum/@of: Support let values

It's not good that c:let is still doing its own logic for this.

* src/current/compiler/js-calc.xsl (compile-calc)[c:sum[@of]]: Support c:let
  values.

src/current/compiler/js-calc.xsl

@@ -202,6 +202,8 @@
 
     <variable name="func" select="ancestor::lv:function" />
 
+    <!-- XXX: this needs to use compile-calc-value, but can't right now
+         beacuse it's not a c:value-of! -->
     <variable name="value">
       <choose>
         <!-- is @of a function param? -->
@@ -216,6 +218,11 @@
           <value-of select="@of" />
         </when>
 
+        <!-- let expression -->
+        <when test="$of = ancestor::c:let/c:values/c:value/@name">
+          <value-of select="$of" />
+        </when>
+
         <!-- maybe a constant? -->
         <when test="
             root(.)/preproc:symtable/preproc:sym[