Crate doesn't compile without "json" feature due to unconfigurable usage of serde_json

[AdamGS]

While the "json" feature is optional and serde_json is marked as an optional dependency, its actually a required one because its used across the crate to format some errors, making it impossible to disable.

[yassun7010]

Indeed!

I tried to move json to features for consistency in module structure, but there was an omission.

I will move it back to required.