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`.
Eventually, there will probably be a `crate` visibility specifier that
does the same thing as `pub(crate)`. This commit replaces `pub` with
`pub(crate)`, so when `crate` is available we can easily switch to it.
`env_var(key)` looks up the value of the environment variable with name `key`, aborting execution if it is not found.
`env_var_or_default(key, default)` looks up the value of the environment variable with name `key`, returning `default` if it is not found.
