Make the autoloader swappable

Right now we're stuck with whichever autoloader Drupal 8 ships with, but we know that the autoloader performance is very bad out of the box. There's the APC switch but there are other options like class maps etc. which people want to experiment with.

I think we should look at making the autoloader swappable (via settings?) and then if necessary adding our own interface so that namespaces etc. can be added consistently. Right now any patch to change the autoloader has to change several lines of code deep in bootstrap, that could likely move to an adapter class instead.