Update contact email address in package headers.

Fix bug in detection of nodes corresponding to unmodified buffers.

Fix URLs in package headers.

Fix potential insidious list creation bugs.

Fix how undo-tree-viualizer-timestamps|diff are set buffer-local.

Major-mode fundctions call `kill-all-local-variables' at start, so these need
to be made buffer-local *after* setting the major-mode to
undo-tree-visualizer-mode, not before. We now set them buffer-local in
undo-tree-visualizer-mode, which seems the appropriate place.

(Thanks to Martin Joerg for flagging this.)

Add missing Package-Requires header for queue dependency.

Implement new undo-tree save format.

Works around Emacs bug #27779: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=27779

New save format serialises the undo-tree, instead of directly writing out the
Elisp object. This avoids triggering a stack overflow in the Emacs Lisp reader
when loading large undo-tree history files, due to the Elisp reader not coping
with deeply nested Elisp objects.

Fix argument order in gv setter definitions.

Switch from cl to cl-lib.

Fix hook function issue in Emacs 27.

Fix some byte-compilation warnings.

Don't attempt to save undo history if history file is unwritable.

undo-outer-limit can also be null (no limit) in recent Emacsen.

Bump version number.

Redraw visualizer when history-discarding invalidates it.

Implement non-recursive undo-tree copier, to avoid hitting Elisp and c stack space limits.

Temporarily increase max-lisp-eval-depth and max-specpdl-size when copying undo-trees.

Proper fix to copy-undo-tree by writing bespoke copier.

Previous fix didn't copy current node correctly.

Fix copy-undo-tree by forcing vector rather than native defstruct.

Fixes broken undo-tree history saving in recent Emacsen that implement native
defstruct types.

(Better solution would be to implement bespoke undo-tree copier, rather than
relying on copy-tree.)

Bump copyright year in readiness for new release.