Move to `orjson`

UltraJSON has been put in to "maintenance-only" status and the maintainers recommend upgrading to [orjson](https://github.com/ijl/orjson). orjson is quite a bit faster than ujson, but does have some API incompatibilities.

I'm working on a project right now where we are caching lots of SpaCy objects, and the numerous calls to `json_dumps` can slow things down. orjson looks to be about twice as fast as ujson on average, and would make a big difference in our case!