fix(derive): require Default trait bound for the skipped field type, rather than for its type parameters

[DanikVitek]

Fixes #339

[DanikVitek]

Now that I think about it, it seems, that the same change could be made for the requirements of BorshDeserialze/BorshSerialize traits themselves. Meaning, that these traits should be required not for the type parameters, but for the field types. Because, once again, some types might implement these traits manually and not need the trait bounds for the generic types. Akin PhantomData<T>