Using no_result_var in Journey's parser generator

Previously the generated parser had an intermediate local variable
`result` that really useful if you're building up a stateful object but
Journey always discards the result argument to the reduce functions.

This produces a simpler parser for anybody who actually wants to read
the thing.

Sadly, there's no real performance speedup with this change.
Merge pull request #16386 from JackDanger/journey-no-result-var

Using no_result_var in Journey's parser generator