ceafbad81f
It mirrors `ExprKind::Binary`, and contains a `BinOpKind`. This makes `AssocOp` more like `ExprKind`. Note that the variants removed from `AssocOp` are all named differently to `BinOpToken`, e.g. `Multiply` instead of `Mul`, so that's an inconsistency removed. The commit adds `precedence` and `fixity` methods to `BinOpKind`, and calls them from the corresponding methods in `AssocOp`. This avoids the need to create an `AssocOp` from a `BinOpKind` in a bunch of places, and `AssocOp::from_ast_binop` is removed. `AssocOp::to_ast_binop` is also no longer needed. Overall things are shorter and nicer.