Fix serde implementation

[Neotamandua]

Summary

The current serde implementation is inconsistent with our other serialization & deserialization implementationse. This is especially important for Scalar which is also in some events on execution-core that support serde json representations.

Possible solution

Examine the differences in how the types of this crate implement Serialize & Deserialize with serde vs. the other serde serialization implementations we now have - and align them properly.