Optimization for strings and more

[kkocdko]

• Reduce the loops in fn parse_group_message.
• Fold code in fn decode_group_message_packet.
• Optimize the convert operation, Vec<u8> to String.

[kkocdko]

I have never seen any fallback...

[kkocdko]

Will reconsider about the fn parse_group_message...

[kkocdko]

Problem fixed.