the `core20` base does not contain lua 5.4.

fallthru was not working properly for overlapping multi-input key
bindings. for example, if the current kmap (with fallthru allowed)
contained a key binding for `a x`, and its parent contained a key
binding for `a y`, then inputting `a y` would not execute the binding
in the parent kmap.

an additional issue arises here in that if the binding for `a y` is
`cmd_insert_data` then both inputs should count as input data, not
just the tail. (usually the inputs leading up to a multi-input
binding are simply consumed and forgotten.) this requires some
gymnastics. it would be a good exercise to see if we can
consolidate `pastebuf` and `input_trail`.

Signed-off-by: Francois Berder <18538310+francois-berder@users.noreply.github.com>

also deprecate `M-[` and `M-]` which start many common escape codes.

consider this an unstable commit. termbox2 is a
line-by-line rewrite of termbox with some extra features,
more flexibility, stricter error handling, and a more
consistent api. it's also redesigned as a single-file
header library. this makes it easy to copy-paste into
other projects, which is what i've done here. at the time
of writing, the canonical termbox2 repo is here:

https://github.com/termbox/termbox2

embarrassingly i don't remember how to reproduce this bug.
i fixed it a while ago and didn't have time to write up a
test case.

doubly-linked lists in utlist keep a tail pointer at
`head->prev`. i often forget this detail and it leads to
bugs such as in this case. commit 9c66fa60 was a partial
fix.

`buffer.actions` is a doubly-linked list, so
`action_to_(undo|redo)` could potentially point to itself
if it is the only item in the list. current coarse undo
code will fall into an infinite loop by failing to check
for this. this patch fixes.