Right-size the map array in PartialEvaluator_readToUnicode

We can avoid a lot of intermediate resizings, by directly allocating the required number of elements for the `map` array.

src/core/evaluator.js

@@ -1380,7 +1380,7 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
         if (cmap instanceof IdentityCMap) {
           return new IdentityToUnicodeMap(0, 0xFFFF);
         }
-        var map = [];
+        var map = new Array(cmap.length);
         // Convert UTF-16BE
         // NOTE: cmap can be a sparse array, so use forEach instead of for(;;)
         // to iterate over all keys.