Fix parsing of `use_file` (#2146)
Fix parsing of `use_file` (#2146)

Because the `use_file` parser definition is recursive, we were applying
the operator swapping mappers more than once in the cases where the rule
recursed.

Instead, we should only apply the mapper to the final result of parsing
the `use_file` rule.

Fixes https://github.com/facebook/reason/issues/1965
Fixes https://github.com/facebook/reason/issues/1980
facebook/reason,facebook/reason,facebook/reason,chenglou/reason,chenglou/reason,chenglou/reason