feat: use EnumsAsInner to extract enum variants

[echou]

It is quite boring to extract specific enum variant using pattern match. I would like to derive all enums with EnumsAsInner. So I can use expr.as_function(), expr.as_ident() ... much simpler than if let Expr::Function(f) = expr { ... }

[alamb]

Possibly related #1204

@tisonkun started doing this for a few types. Maybe we can work on a few more common variants for the next release