Implement Copy for the primitive value deserializers

[dtolnay]

https://docs.serde.rs/serde/de/value/index.html

The primitive deserializers (I8Deserializer, I16Deserializer, etc) only wrap only wrap a value of their primitive type and a phantom data for the error type, so it should be fine to allow copying.

[dtolnay]

As part of this it would be good to remove the E: Clone bounds inferred by derive(Clone). The bound is not required because these types only store PhantomData<E>.

- impl<E: Clone> Clone for BoolDeserializer<E>
+ impl<E> Clone for BoolDeserializer<E>