Lazily print backtrace frame local variables

Lazily print backtrace frame local variables

Instead of printing the local variables for all frames when the
backtrace buffer is created, print them when they are first made
visible.  Add a prefix argument to backtrace-toggle-locals to toggle
local variables display for the entire buffer.
* lisp/emacs-lisp/backtrace.el (backtrace-view): Mention
:show-locals in docstring.
(backtrace-get-section-end): Remove function.
(backtrace-toggle-locals): Add prefix argument.
(backtrace--with-output-variables): Move before first use.
(backtrace--set-frame-locals-visible): New function.
(backtrace--set-locals-visible-overlay): New function.
(backtrace--set-locals-visible): Remove function.
(backtrace-toggle-feature): Remove TODO comment.
(backtrace--print-locals): Skip printing the locals if they are not
visible.