[development] Strip out theme functions into a .theme file

[Khalid B]

> >There is lots of subtle trade-offs involved (eg. less PHP parsing,
> >smaller memory footprint, but more fopen() calls) and it is hard to
> >predict how these level out.  Let's not make claims and just
> >benchmark it?
>
> Would an opcode cache help to negate the fopen() calls? It may be worth
> benchmarking to see.

It may help, or may not.

The point is: many people use Drupal on a shared host where there is
no op-code cache. We cannot REQUIRE an op-code cache and
leave these people hapless ...