v0.1.5 [pickle]

Make All objects in symusic pickable

• Add c++20 in github actions workflow @lzqlzzq
• Add using zpp_bits as pickle backend @Yikai-Liao
  • pickle for symusic is now 10 times faster than loading from raw midi files
  • Just use pickle for multi-processing and cache. Don't use pickle to store midi for a long time, the format might be changed in the future
  • There is still a redundant copy in pickle serialization process, may try py::buffer_info to achieve zero_copy
• add tpq construtor in ScoreFactory @Yikai-Liao
• add PedalFactory @Natooz
• add ttype readonly property for all classes @Yikai-Liao