Yet another Uxn core, this time as an OCaml library.

It has no dependencies, and depends on OCaml >=5.3 for its effect syntax.

It was made for use in Llop, a concatenative language targetting the Uxn virtual
machine, but can be used for other purposes. See `exe/uxnemu.ml` for a minimal
Uxn/Varvara emulator that is able to run programs like Drifloon.