2021-08-01 version 4.0.0

• Fix and improve alignment logic (#962)
• Fix iovec name conflict (#953)
• Fix empty string print (#942)
• Fix buffer ptr size (#899)
• Fix UB. Check null pointer before using memcpy() (#890)
• Improve CI environment (#885, #899)

<< breaking changes >>

• Separate C part of the msgpack-c from C/C++ mixed msgpack-c (#876, #878)