support serde values

[yanns]

When we already have Value, we can avoid using the Serialize trait. This allows us:

• to avoid the usage of two panic!
• to have faster code. No need to serialize the whole tree.

As an alternative, we could also make the following function public:

pub(crate) fn diff<'a>(lhs: &'a Value, rhs: &'a Value, config: Config) -> Vec<Difference<'a>>