Post #1629769

@zwarich `VarError` (analogous to your `Expr`) contains `VarErrorKind` (your `ExprKind`) contains `VarKind` (a bare enum that has an unfortunate name collision). https://github.com/thaliaarchi/mini-alive/blob/1bb46e714e1772b6c12ad7773c90361133cbdbab/src/syntax/error.rs#L129-L174