Srsly

* Fix length limits on `msgpack` serialization.

* Move `ujson` to `srsly.ujson` for API consistency.
* Add pickle utilities: `pickle_loads` and `pickle_dumps`.
* Add msgpack utilities: `msgpack_loads`, `msgpack_dumps`, `write_msgpack`, `read_msgpack`.
* Improve test suite.

* Expose `json_dumps` and `json_loads` helpers.
* Add support for `sort_keys` via `json`.

* Fix tests across Python versions and platforms.
* Improve file path handling.