Fix exponential behavior in accent production (#834)

src/functions.js was returning two properties referring to the base; since buildHTML runs `JSON.parse(JSON.stringify(tree))` to get an immutable copy, that meant we'd traverse and serialize and parse an exponentially-sized tree.

Test Plan: `\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{\breve{A}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}` renders instantly; previously, it reliably timed out with even half that depth.

src/buildHTML.js

@@ -1576,7 +1576,7 @@ groupTypes.horizBrace = function(group, options) {
 
 groupTypes.accentUnder = function(group, options) {
     // Treat under accents much like underlines.
-    const innerGroup = buildGroup(group.value.body, options);
+    const innerGroup = buildGroup(group.value.base, options);
 
     const accentBody = stretchy.svgSpan(group, options);
     const kern = (/tilde/.test(group.value.label) ? 0.12 : 0);

src/functions.js

@@ -656,7 +656,6 @@ defineFunction([
         label: context.funcName,
         isStretchy: isStretchy,
         isShifty: isShifty,
-        value: ordargument(base),
         base: base,
     };
 });
@@ -677,7 +676,6 @@ defineFunction([
         label: context.funcName,
         isStretchy: false,
         isShifty: true,
-        value: ordargument(base),
         base: base,
     };
 });
@@ -704,12 +702,11 @@ defineFunction([
 ], {
     numArgs: 1,
 }, function(context, args) {
-    const body = args[0];
+    const base = args[0];
     return {
         type: "accentUnder",
         label: context.funcName,
-        value: ordargument(body),
-        body: body,
+        base: base,
     };
 });
 

src/stretchy.js

@@ -442,6 +442,14 @@ c-1 5-5 9-11 9h-2L532 67 19 159h-2c-5 0-9-4-11-9l-5-22c-1-6 2-12 8-13z'/>`,
 <line x1='0' y1='100%' x2='100%' y2='0' stroke-width='0.046em'/>`,
 };
 
+const groupLength = function(arg) {
+    if (arg.type === "ordgroup") {
+        return arg.value.length;
+    } else {
+        return 1;
+    }
+};
+
 const svgSpan = function(group, options) {
     // Create a span with inline SVG for the element.
     const label = group.value.label.substr(1);
@@ -453,7 +461,7 @@ const svgSpan = function(group, options) {
     if (utils.contains(["widehat", "widetilde", "undertilde"], label)) {
         // There are four SVG images available for each function.
         // Choose a taller image when there are more characters.
-        const numChars = group.value.value.length;
+        const numChars = groupLength(group.value.base);
         if (numChars > 5) {
             height = 0.312;
             imageName = (label === "widehat" ? "widehat" : "tilde") + "4";