Modifies parsing to return strongly-typed `Thunk`s, which contain both
the function implementation, as well as the correct number of arguments.
This moves unknown function and function argument count mismatch errors
to parse time.
Make analysis resolve recipe dependencies from names (`Name`) to recipes
(`Rc<Recipe>`), to give type-level certainty that resolution was performed
correctly and remove the need to look up dependencies on run.
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.
