It's been around two and a half years, and many versions, since this
warning was first introduced, so it feels reasonable to finally turn it
into a hard error. It will remain a special-cased error for a little
while.
Just's first parser performed both parsing, i.e the transformation of a
token stream according to the language grammar, and a number of consistency
checks and analysis passes.
This made parsing and analysis quite complex, so this diff introduces a
new, much cleaner `Parser`, and moves existing analysis into a dedicated
`Analyzer`.
