Improve performance of field.html.twig

Latest benchmarks from http://drupal.org/node/1898062#comment-7444854

=== field-base-8.x..field-base-8.x compared (519e62c02d767..519e633ddef65):

ct  : 151,892|151,892|0|0.0%
wt  : 582,054|581,661|-393|-0.1%
cpu : 563,711|563,090|-621|-0.1%
mu  : 16,435,144|16,435,144|0|0.0%
pmu : 16,877,640|16,877,640|0|0.0%

http://www.lionsad.de/xhprof-kit/xhprof/xhprof_html/?run1=519e62c02d767&...

=== field-base-8.x..1898062-86-field compared (519e62c02d767..519e636f9416a):

ct  : 151,892|158,027|6,135|4.0%
wt  : 582,054|598,276|16,222|2.8%
cpu : 563,711|578,815|15,104|2.7%
mu  : 16,435,144|16,492,104|56,960|0.3%
pmu : 16,877,640|16,934,536|56,896|0.3%

http://www.lionsad.de/xhprof-kit/xhprof/xhprof_html/?run1=519e62c02d767&...