replace `serde_json` dev-dependency for `serde_test`

[alejandro-vaz]

currently only used in one test in tests.rs to verify the implementation of serialization/deserialization

it should be replaced by a handwritten small serializer-deserializer that does not require an additional dependency and just checks correctness

[alejandro-vaz]

this is going to be very cumbersome, writing a serialiazer-deserializer is too big for the scope of the test

[alejandro-vaz]

we can use serde_test instead that removes many dependencies, and makes it simpler to test

[alejandro-vaz]

merged