Fix handling of depth

[etiennebarrie]

This adds a few tests around the handling of depth when an exception (in particular JSON::NestingError) is raised during JSON generation. The core idea is that state.depth shouldn't be changed when calling state.generate(obj) or obj.to_json(state). While we can't be sure in the last case for user-controlled classes, we can reasonably ensure that it works with Hash and Array.

For CRuby, the approach is to never use the current State object, and always start generation with a generate_json_data::vstate set to Qfalse. It's only the first time we need to call to_json, that we create a temporary State which can be freely mutated by user code, since we never copy depth back from State to generate_json_data. All subsequent calls to to_json will use the same State, setting its depth to the correct value at that point.

Since State is no longer mutated, we don't need State#generate_new anymore.