2020-06-05 version 3.3.0

• Add json example for C (#870)
• Add both header and body packing functions for C (#870)
• Set default ref_size and chunk_size to vrefbuffer (#865)
• Add examples (#861)
• Improve build system (#839, #842)
• Improve tests (#829)
• Improve documents (#828)
• Remove some warnings (#827, #851, #871)
• Improve CI environment (#824, #831, #833, #834, #846, #860, 874)