display-backtrace if it wasn't explicitly given by the user.
(write-frame-long/application): Also print corresponding source
expression.
("evaluate"): Evaluate in local environment frame, if existent;
Handle errors.
both forward and backward views of the stack
(write-frame-index-short, write-frame-index-long):
Use selector `frame-number';
(select-frame-absolute): Use frame-number->index.
("backtrace"): Use builtin backtrace printing.
Use (ice-9 debug).
Use readline conditionally.
