Allow trait boundaries for generic newtypes

[greyblake]

The following type must be possible to declare:

#[nutype(
    sanitize(with = |v| { v.sort(); v }),
    derive(Debug)
)]
struct SortedVec<T: Ord>(Vec<T>)

• Make sure to have a proper test coverage
  • without validation
  • with validation
• Make sure Serialize and Deserialize can be derived
• Add the example to the README and docs
• Add the example to any_generics example project