0.3.0 Major refactoring

Added

• Non-blocking Iterator access for ShmReader
• The exhausted method in ShmReader which tell us if a channel still provide records or not.
• Major refactoring of the various modules

Changed

• Header struct was renamed Metadata
• Error handling in ShmWriter
• Encodable trait returns Result
• The read method in Reader was renamed try_read, requires no callback handler, and returns an Option wrapped in a Result

Removed

• The codecs subcrate
• The core subcrate
• DataFormat structure