Arbitrary call stack access for external debuggers changeset.

[John W. Eaton]

On  3-Mar-2008, Ryan Rusaw wrote:

| I sent in this patch in prior to the 3.0.0 release, but it never made
| it in prior to the switch to mercurial.
| 
| 2008-03-03  Ryan Rusaw  <rrusaw at gmail.com>
| 
| 	* top_lev.h: Replace internal std::list with stl::deque in
| 	octave_call_stack to allow access to elements by position
| 	index via element_at () call.
| 	* top_lev.cc: Implement new element_at () call for updated
| 	octave_call_stack class
| 	* pt-stmt.h: Declare new tree_statement_stack class modelled
| 	after octave_call_stack class
| 	* pt-stmt.cc: Remove curr_statement and curr_caller_statement,
| 	and implement tree_statement_stack class to allow call stack
| 	statements to to be accessed by depth index.
| 	* error.cc: Replace references to curr_statement to
| 	tree_statement_stack::current ()
| 	* input.cc: Replace references to curr_statement to
| 	tree_statement_stack::current ()
| 	* ov-usr-func.cc: Remove references to curr_statement and
| 	curr_caller_statement

I applied this patch with some changes.

Thanks,

jwe