Instead of having a generaic main loop in be_main with several callbacks
into the targets, split the code in be_main up into a set of convenience
functions and let the targets implement the main loop itself. This is
nicer because:

- The control flow into and out of the functions in the target becomes
  obvious as no external party is using callbacks anymore.
- Targets can insert custom code at any place between the generic
  passes.
- In the future we can parameterize the generic backend passes directly
  instead of stuffing info into generic isa_if structures.