idea: Newtype wrapper for open enums

`AsReprInherent` creates the possibility of a newtype adapter to create an open type (all bit patterns valid) from a closed type (some bit patterns invalid):

```rust
#[repr(transparent)]
pub struct Open(pub T::InherentRepr)
where
T: AsReprInherent,
::InherentRepr: Integer;
```

Could automatically get `TryFrom> for T where T: TryFromRepr {}` and `From for Open where T: ReprFrom`