Fix %hela, %nara, %lose hints

[ashelkovnykov]

The %nara and %hela hints might be useful during debugging and testing for checking the state of the mean stack. However, %nara is currently unimplemented and %hela is implemented incorrectly: %hela prints the mean stack to the bottom of the nearest frame, but this is not guaranteed to be the entire mean stack for the event, nor even for the current level of virtualization.

In addition, the %lose hint would also be useful, as we currently do no pruning of the mean stack, meaning that massive traces are possible.