Closes#592
A new node is added in `desugared`, and translated into an exploded structure
literal during translation to `scopelang`. The main reason to put it there is
that it needs to be after disambiguation, since that is used to discover the
type of the structure that is being updated.
Ensuring messages don't print overlong lines still requires some manual work:
- if they don't contain any `Format` directives (`%` or `@`), use `"%a"
Format.pp_print_text` to turn word-wrapping on.
- otherwise replace spaces with `@ ` to mark possible cutting points, as soon
that it's possible the line will get over 80 chars (most often, this means
starting before the first `%a`)
Lots of tests have a new warning because they were calling subscopes without
using their outputs. A better solution could be to mark these subscopes as
`output`, now that it's possible !
