[Eva] Small changes to the new Callstack interface.

- The callstack type is not private anymore. Only the current callstack in
  Eva_utils should not be modified without using push/pop; everywhere else,
  functions can freely create callstacks if needed.
  Removes function [change_thread], as it is now useless.

- [pop] only returns the new callstack: this simplifies all its current uses.

- new function [last_caller] returning the function from the penultimate call.

- new function [pretty_debug] to allow using Hptmap on callstacks.