Arguably missed in #769
Note, this is reminding me we have more class members to rename for `_`, but I felt it's best to use the new naming instead of adding more to clean up.
With this, only main.cpp instantiates an arena. Maybe we'll want to split that up more later (e.g., so that the runtime interpreter uses its own arena), but given the intent to have type-checking update the AST, I thought this was a reasonable approach for now in order to avoid ownership complexities.
Fixes#769
Jon Meow
