Support cloning

It would be very useful if the types provided by this crate supported cloning. This way structs could simply derive Clone:
```rust
#[derive(Clone, Debug, Deserialize)]
struct Human {
    name: NonEmptyString,
    age: MinMaxU8<18, 80>,
    friends: NonEmptyVec<String>,
}
```

This is actually the only reason why I'm currently using refined_type - because I need to clone my data.