Previously, string JSON values were special cased to be provided as
bare strings, which means that CFFI modules have to either know what
type each value is expected to be, or use a heuristic such as trying to
decode and then treating the value as a string on failure.
Instead, we can always return JSON, and let the downstream consumer
handle deserialising the value into whatever type is expected.
The new behaviour is gated on a new ABI version 2: modules built against
version 1 will continue to get the old behaviour.
