Macro impl_newtype

macro_rules! impl_newtype {
    ($newtype:ty, $inner:ty) => { ... };
}

Implements the following traits for a newtype of a der decodable/encodable type:

• From conversions to/from the inner type
• AsRef and AsMut
• DecodeValue and EncodeValue
• FixedTag mapping to the inner value’s FixedTag::TAG

The main case is simplifying newtypes which need an AssociatedOid