BigInt fixes and unit tests

[AlexanderKolberg]

This fixes two issues

1. Nested arrays weren't decoding correctly - Arrays like Message[] with BigInt fields failed because the [] suffix wasn't stripped during recursion
2. JsonEncode was mutating original request objects - Switched to using a custom replacer function with JSON.stringify instead, which is much simpler and doesn't mutate the caller's object. This removes the need to manually transverse the object, and removes the need for the BIG_INT_FIELDS when encoding (its still needed for decoding), it will just encode every Value with type of Bigint as string

Added unit tests for the above. in a separate directory for now, will move them with the other tests in a follow-up PR

[pkieltyka]

discussed with Alex, looks good