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.
Optimize common cases in hexToStr().

This avoids the creation of over two million array objects when viewing
http://www.dynacw.co.jp/Portals/3/fontsamplepdf/sample_4942546800828.pdf,
and reduces load time from 76 to 73 ms.
Merge pull request #5071 from nnethercote/font-savings

Optimize a font-heavy document