bug#24514: 24.5; [WIP][PATCH] Lispy backtraces

[Eli Zaretskii]

> From: Clément Pit--Claudel <clement.pit@gmail.com>
> Date: Thu, 22 Sep 2016 22:22:13 -0400
> 
> I'm not sure what the right way to transition is.  Maybe Emacs should let 
> Lisp programs access the backtraces in a structured way, and then backtrace 
> printing would only be a user-facing facility (programs wouldn't use the 
> textual representation).

How about if we install this with a user option, off by default, that
switches to the new format when turned on?  Then we could collect user
experience for some time, and make this the default if everyone likes
it.