Flatten the Frame tree. Now each function call produces a
single frame and non-overlapping variables reuse frame slots.
As a result, entering and exiting blocks no longer requires
code execution, which means jumps across block boundaries
should be doable now. Frame slot initialization happens at
definition time now, instead of at frame creation time. As an
added bonus, Scope's are now exclusively compile-time objects
and we no longer need to specially track the function
activation frame for access to out vars.