Add into_* methods for syntax nodes codegen

[A4-Tacks]

Example

impl Adt {
    pub fn into_enum(self) -> Option<Enum> {
        if let Self::Enum(variant) = self { Some(variant) } else { None }
    }
    pub fn into_struct(self) -> Option<Struct> {
        if let Self::Struct(variant) = self { Some(variant) } else { None }
    }
    pub fn into_union(self) -> Option<Union> {
        if let Self::Union(variant) = self { Some(variant) } else { None }
    }
}