Add backtrace-mode and use it in the debugger, ERT and Edebug

Add backtrace-mode and use it in the debugger, ERT and Edebug

* doc/lispref/debugging.texi (Using Debugger): Remove explanation of
backtrace buffer.  Refer to new node.
(Backtraces): New node.
(Debugger Commands): Refer to new node.  Remove 'v'.
* doc/lispref/edebug.texi (Edebug Misc): Refer to new node.
* doc/misc/ert.texi (Running Tests Interactively): Refer to new node.

* lisp/emacs-lisp-backtrace.el: New file.
* test/lisp/emacs-lisp/backtrace-tests.el: New file.

* lisp/emacs-lisp/debug.el: (debugger-buffer-state): New cl-defstruct.
(debugger--restore-buffer-state): New function.
(debug): Use a debugger-buffer-state object to save and restore buffer
state.  Fix bug#15749 by leaving an unused buffer in debugger-mode,
empty, instead of in fundamental-mode, and then when reusing a buffer,
not calling debugger-mode if the buffer is already in debugger-mode.
(debugger-insert-backtrace): Remove.
(debugger-setup-buffer): Use backtrace-mode.
(debugger--insert-header): New function.
(debugger-continue, debugger-return-value): Change check for flags to
use backtrace-frames.
(debugger-frame-number): Determine backtrace frame number from
backtrace-frames.
(debugger--locals-visible-p, debugger--insert-locals)
(debugger--show-locals, debugger--hide-locals)
(debugger-toggle-locals): Remove.
(debugger-mode-map): Make a child of backtrace-mode-map.  Move
navigation commands to backtrace-mode-map.  Bind 'q' to debugger-quit
instead of top-level.  Make Help Follow menu item call
backtrace-help-follow-symbol.
(debugger-mode): Derive from backtrace-mode.
(debug-help-follow): Remove.  Move body of this function to
'backtrace-help-follow-symbol' in backtrace.el.
(debugger-quit): New function.

* lisp/emacs-lisp/edebug.el (edebug-unwrap-results): Remove warning
in docstring about circular results.
(edebug-unwrap): Use pcase.
(edebug-unwrap1): New function to unwrap circular objects.
(edebug-unwrap*): Use it.
(edebug--frame): New cl-defstruct.
(edebug-backtrace): Call the buffer *Edebug Backtrace* and use
backtrace-mode.  Get the frames from edebug--backtrace-frames.
(edebug--backtrace-frames, edebug--unwrap-and-add-info)
(edebug--symbol-not-prefixed-p): New functions.

* lisp/emacs-lisp/lisp-mode.el
(lisp-el-font-lock-keywords-for-backtraces)
(lisp-el-font-lock-keywords-for-backtraces-1)
(lisp-el-font-lock-keywords-for-backtraces-2): New constants.

* lisp/emacs-lisp/ert.el (ert--print-backtrace): Remove.
(ert--run-test-debugger): Use backtrace-get-frames.
(ert-run-tests-batch): Use backtrace-to-string.
(ert-results-pop-to-backtrace-for-test-at-point): Use backtrace-mode.
(ert--insert-backtrace-header): New function.

* tests/lisp/emacs-lisp/ert-tests.el (ert-test--which-file):
Use backtrace-frame slot accessor.