Think of cross-platform roundtrip property tests

[chshersh]

It would be nice to have the roundrip property tests that will do the following steps for each data type:

1. Generate random value on backend.
2. Encode to JSON on backend.
3. Send to frontend.
4. Decode from JSON on frontend.
5. Encode to JSON on frontend.
6. Send it back to backend.
7. Decode from JSON on backend.
8. Compare with the original value.

Now, I'm pretty sure nobody did this before so we might have difficulties implementing this complex architecture. As a first step instead of doing property-based tests we can harcode several values and just display them both on frontend and backend.

[chshersh]

The part with testing hardcoded value is done in #54 and it works 👍