Optimize template_preprocess_field()

Spin-off from #659788: [meta issue] theme('field') is too slow, which is inhibiting its more widespread use. One of several patches aimed at optimizing theme('field') while still leaving it as a template implementation rather than a function implementation. Will provide benchmarks once #538164: Comment body as field lands, since that's the driving use-case.

This patch is the same as #659788-8: [meta issue] theme('field') is too slow, which is inhibiting its more widespread use, but I want to convert that issue into a meta-issue, so moving the patch here.

In addition to some optimization, this patch includes minor cleanup that yched approved of in #659788-10: [meta issue] theme('field') is too slow, which is inhibiting its more widespread use.