Enum Ptr 0.2.0 Plan

[QuarticCat]

• [x] Remove unit variant support and named variant support
• [x] Support generic type params
• [x] Change Aligned::Pointee to Aligned::ALIGNMENT
• [x] Support Rc and Arc
• [x] Rename feature std to alloc
• [x] Rename map_ref_mut to map_mut
• [ ] Add abilities to use high bits
• [x] Add more methods to smooth user experience
• [ ] Deal with the new discriminant value grammar