Reduce the number of isCmd calls slightly in the XRef class

This reduces the total number of function calls, when reading the XRef table respectively when fetching uncompressed XRef entries.
Note in particular the `XRef.readXRefTable` method, where there're *two* back-to-back `isCmd` checks rather than just one.

Author: Snuffleupagus

src/core/obj.js

@@ -20,7 +20,7 @@ import {
   warn
 } from '../shared/util';
 import {
-  clearPrimitiveCaches, Dict, isCmd, isDict, isName, isRef, isRefsEqual,
+  clearPrimitiveCaches, Cmd, Dict, isCmd, isDict, isName, isRef, isRefsEqual,
   isStream, Ref, RefSet, RefSetCache
 } from './primitives';
 import { Lexer, Parser } from './parser';
@@ -1200,10 +1200,15 @@ var XRef = (function XRefClosure() {
           entry.gen = parser.getObj();
           var type = parser.getObj();
 
-          if (isCmd(type, 'f')) {
-            entry.free = true;
-          } else if (isCmd(type, 'n')) {
-            entry.uncompressed = true;
+          if (type instanceof Cmd) {
+            switch (type.cmd) {
+              case 'f':
+                entry.free = true;
+                break;
+              case 'n':
+                entry.uncompressed = true;
+                break;
+            }
           }
 
           // Validate entry obj
@@ -1685,7 +1690,7 @@ var XRef = (function XRefClosure() {
       if (!Number.isInteger(obj2)) {
         obj2 = parseInt(obj2, 10);
       }
-      if (obj1 !== num || obj2 !== gen || !isCmd(obj3)) {
+      if (obj1 !== num || obj2 !== gen || !(obj3 instanceof Cmd)) {
         throw new XRefEntryException(`Bad (uncompressed) XRef entry: ${ref}`);
       }
       if (obj3.cmd !== 'obj') {