I've set `lexical-binding` to `nil` in all Emacs Lisp files to suppress
the warnings introduced in Emacs 31 requiring all elisp files to have a
`lexical-binding` comment.
This retains the default behavior of dynamic binding when no
`lexical-binding` comment is present. With it set to `t` across the
board, various things break, and fixing those is a task for another day.
Turns out undo-tree massively increases undo limits, and the defaults
seem to cut off history for me much sooner than I'd like. So I set the
limits to the same high values are undo-tree does.
Also replace undohist with undo-fu-session.
Overall this seems to yield a more reliable undo history, as undo-fu and
vundo supposedly operate with vanilla emacs undo history state, while
undo-tree supposedly does some custom tweaks to the undo state.
