eliminate direct access to call stack from evaluator The call stack is an internal implementation detail of the evaluator. Direct access to it outside of the evlauator should not be needed. * pt-eval.h (tree_evaluator::get_call_stack): Delete.

## scope0.m
function scope0
  C;
  function A
    B;
    function B
    endfunction
  endfunction

  function C
    D;
    function D
      A;
    endfunction
  endfunction
endfunction