optimize template_preprocess_field()

Looking at http://rugbybreakdown.net/xhprof/xhprof_html/?run=4b4e34ee2a91c&symbol=t...

- element_children() is 24% of template_preprocess_field() - we can use a simpler alternative here.
- array_merge() is another 10% - a simple array addition will do.

I'm actually not sure a series of

$variables['foo'] = 'bar';
$variables['baz'] = 'goo';

would be faster than

$additions = array(
'foo' => 'bar',
'baz' => 'goo',
);
$variables = $addition + $variables;


Possibly microptimization since that same profling info only credits t_preprocess_field() with 1% of page load - although I'm not sure the test site is to date with recent template discovery optimizations, which mechanically raise the % cost of other funcs.