Prologue analysis interface

[Jim Blandy]

Daniel, you said that you felt that the interface in the
prologue-value.h that I posted wasn't at the right level, and
suggested that, instead of providing types and some operations on
those types, it should instead provide the interpretation loop and
call back into tdep code to interpret specific instructions.

Could you sketch that interface for me?  I'm concerned that it will be
difficult to allow the tdep code the flexibility it needs to model the
processor without a lot of bureaucracy, but I'm willing to be
impressed.