`MacroM` will implement `MonadRef` because
1- It will be easier to throw errors from macros
2- We will be able to `getRef` to retrieve the syntax node at macro
rules.
I renamed `Ref` to `MonadRef` to make it consistent with other classes
providing monadic methods (e.g. `MonadEnv`, `MonadState`, etc).
cc @Kha
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