Bugfix release 3.0.1

Critical fixes:

• temp_msgpack_gc/flatten race (#109)
• stack restore for validate error handling (#11)
• schema evolution for nullable fields (#76)

Fixes:

• installation via luarocks (6fbd4d6)

Features:

• allow default values for records, nullable records, unions (#99, 595fe70, 9d853a7)

Extra:

• code refactoring
• add comments to codebase
• deleted unused register (ripv)
• extend documentation