Xorjson

Fixed

- Fix `orjson.dumps(..., **{})` raising `TypeError` on python3.6.

Added

- `orjson.OPT_APPEND_NEWLINE` appends a newline to output.

Changed

- Improve deserialization performance of `str`.

Fixed

- Fix serializing zero-dimension `numpy.ndarray`.

Fixed

- Fix repeated serialization of `str` that are ASCII-only and have a legacy
(non-compact) layout.

Added

- `orjson.OPT_PASSTHROUGH_SUBCLASS` causes `orjson.dumps()` to pass
subclasses of builtin types to `default` so the caller can customize the
output.
- `orjson.OPT_PASSTHROUGH_DATETIME` causes `orjson.dumps()` to pass
`datetime` objects to `default` so the caller can customize the
output.

Changed

- `orjson.dumps()` does not serialize `dataclasses.dataclass` attributes
that begin with a leading underscore, e.g., `_attr`. This is because of the
Python idiom that a leading underscores marks an attribute as "private."
- `orjson.dumps()` does not serialize `dataclasses.dataclass` attributes that
are `InitVar` or `ClassVar` whether using `__slots__` or not.