Minimize memory usage of font-related arrays.

This patch replaces some vanilla arrays with typed arrays, and avoids
some array copying.

It reduces the peak RSS when viewing
http://www.dynacw.co.jp/Portals/3/fontsamplepdf/sample_4942546800828.pdf
from ~940 MiB to ~750 MiB, and reduces its load time from 83 to 76 ms.

src/shared/util.js

@@ -436,7 +436,7 @@ function bytesToString(bytes) {
 
 function stringToArray(str) {
   var length = str.length;
-  var array = [];
+  var array = new Uint16Array(length);
   for (var i = 0; i < length; ++i) {
     array[i] = str.charCodeAt(i);
   }