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.