Refactoring of the way that inlinings and rules are activated

Principally, the SimplifierMode now carries several (currently
four) flags in *all* phases, not just the "Gentle" phase.
This makes things simpler and more uniform.

As usual I did more refactoring than I had intended.

This stuff should go into 7.0.2 in due course, once
we've checked it solves the DPH performance problems.