[development] Eliminate hooks entirely in Drupal 6

[Earl Miles]

Sammy Spets wrote:
> +1 from me provided the performance gain is real.
> 

I think the first actual work step would be to do a light implementation which 
does the following things:

o Implements a simple drupal_register_callback
o Changes module_implements and module_invoke appropriately
o Looks for instances where module_invoke is being used improperly
o Identifies some chunk of code to be set aside

And then some benchmarks to be run.

I think the biggest gains will be in theming, in the end, but the conditional 
includes should be able to make a fair chunk of code not be loaded.

One point I'd like to make: Drupal uses a LOT of CPU on many shared hosts. 
Theoretically, if done well, this method could reduce the amount of CPU it uses 
simply by reducing its footprint, especially if some of the larger modules 
(*cough* Views, CCK, ecommerce) are able to restructure to take advantage of it.