Add static caching for Twig_Environment::getTemplateClass()

Coming from http://drupal.org/node/1979290.

```
=== baseline-views-field-twig..views-field-twig-static-cache compared (517896dc57c6e..5178989df1d0f):

ct  : 116,895|115,892|-1,003|-0.9%
wt  : 478,367|474,349|-4,018|-0.8%
cpu : 449,530|444,639|-4,891|-1.1%
mu  : 17,447,208|17,453,200|5,992|0.0%
pmu : 17,658,176|17,664,136|5,960|0.0%
```

http://www.lionsad.de/xhprof-kit/xhprof/xhprof_html/?run1=517896dc57c6e&run2=5178989df1d0f&source=drupal-perf-cottser&extra=baseline-views-field-twig..views-field-twig-static-cache

We found that performance could be measurably improved when the same template is rendered multiple times in a request.