* Fixes incorrect loop `next`ing behaviour
* Includes fix for determining return address

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.

Benefits of this CL:

* Commands no longer rely on closured proc struct
* No two-step process for basic command initializaion

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.