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`.
Previously, warnings upon encountering a deprecated use `=` in
assignments, exports, and aliases would print a message without any
indication of where the offending `=` was. This diff adds a proper
`Warning` enum, and uses it to report context, as is done with
compilation and runtime errors.
