Change name to option, it's too long...
debug_program is already "protected"
Whitespace
Byebug's calculated stacksize is now the real one

There three concepts in the code now:
- calced_stack_size: used internally by byebug to track stack size and
  detect stopping points. It's only exposed to make TracePoint API bugs
  become apparent but it should always equal real_stack_size.
- real_stack_size: it's the real stack size
- stack_size: this is the stack size from byebug's user point of view,
  i.e., the real_stack_size minus the number of byebug's internal
  frames. Used in the cli, but not internally.