Improvements:
* `next`ing through a loop works correctly (when not already within a loop)
* `next`ing out of a function works correctly
Needs work:
* `next`ing in a loop can be improved when starting within a loop
This current implementation does not cover the following:
* Setting correct breakpoint when exiting loop
* Setting correct breakpoint when returning from function
* All facilities are available for this, it just is not taken into
account in the current `next` implementation.
Implement basic api for figuring out, given a current PC value, where
the function will return. Currently the API provides only a way to
determine the offset from SP (the Canonical Frame Address). It is left
up to the caller to grab the actual address from the traced program.
