Right now I'm concerned that the way this generates the code is inefficient, in particular this line:
`in ok @@ (access_path, key', value', loc)`
Since the comments [on my code for the set patch](https://gitlab.com/ligolang/ligo/merge_requests/127)
warned that repeated generation of the access path is a bad idea(?). In any case this does work, so
it's something I can improve on.
