* lisp/emacs-lisp/trace.el: Rewrite, use nadvice and lexical-binding.

* lisp/emacs-lisp/trace.el: Rewrite, use nadvice and lexical-binding.
(trace-buffer): Don't purecopy.
(trace-entry-message, trace-exit-message): Add `context' arg.
(trace--timer): New var.
(trace-make-advice): Adjust for use in nadvice.
Add `context' argument.  Delay `display-buffer' via a timer.
(trace-function-internal): Use advice-add.
(trace--read-args): New function.
(trace-function-foreground, trace-function-background): Use it.
(trace-function): Rename to trace-function-foreground and redefine as
an alias to that new name.
(untrace-function, untrace-all): Adjust to the use of nadvice.