Handle compact inside arrow functions

[sirbrillig]

The code to detect and process variables inside calls to compact() (please, stop using that function 😩) was finding and processing those variables all at once, using the scope of the compact token itself as if it were a variable. However, this does not work for compact used inside arrow functions because there are multiple possible scopes for each variable.

In this PR, we refactor the code to process each variable inside compact() separately, allowing us to determine each one's scope before determining if it is defined.

Fixes #330