Remove drupal_render_children() from drupal_render()

According to the latest benchmarks, the theme system, and the drupal_render pipeline in particular, is one of our larger time sinks. drupal_render() is intensely recursive, and with drupal_render_children() is double-recursive. PHP stack calls are not particularly fast. So let's see if we can save ourselves a few stack calls in drupal_render(), which is along pretty much every critical path now.

The attached patch removes the call to drupal_render_children() from drupal_render() and inlines the same behavior. There should be zero functionality change, but we do save one function call for every time we need to render a child element. I did not remove drupal_render_children() because it's still called from a ton of theme functions, but at least this will reduce the recursion depth.

With APC enabled using ab -n200 -c1 on node 1, Before:

Requests per second:    10.72 [#/sec] (mean)
Time per request:       93.251 [ms] (mean)  
Time per request:       93.251 [ms] (mean, across all concurrent requests)

And After:

Requests per second:    11.07 [#/sec] (mean)
Time per request:       90.373 [ms] (mean)
Time per request:       90.373 [ms] (mean, across all concurrent requests)

Not a massive difference, but every little bit helps. It should also be a bigger difference on sites with more complex node bodies, or deep forms like CCK forms, because those have deeper recursion in the rendering pipeline.