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/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,
     };
 });