Grainfmt: Experiment with removal of infix_wrap argument

[phated]

The infix_wrap argument mostly seems like a workaround and I think we could better factor out the behavior. This is just cleanup on #1976 so we can do it later.

[ospencer]

An idea would be to create a print_infix_expression function which applies the wrapping, and call it from everywhere we pass ~infix_wrap=Fun.id.