v0.2.1

Some renaming to get a cleaner API (aliases with the old names are kept):
- FromTokioAsyncReadJsonReader -> TokioAsyncReaderJsonParser
- FromBufferJsonReader -> SliceJsonParser
- FromReadJsonReader -> ReaderJsonParser
- LowLevelJsonReader -> LowLevelJsonParser
- LowLevelJsonReaderResult -> LowLevelJsonParserResult
- ParseError -> JsonParseError
- SyntaxError -> JsonSyntaxError
- ToTokioAsyncWriteJsonWriter -> TokioAsyncWriterJsonSerializer
- ToWriteJsonWriter -> WriterJsonSerializer<W>
- LowLevelJsonWriter -> LowLevelJsonSerializer
- read_next_event -> parse_next
- write_event -> serialize_event