Packet Writer and Scanner

Created by: LouYu2015

Summary:

Implemented packet writer and scanner to simplify the process of constructing and parsing packets.

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In file .gitignore:

• Removed a duplicated .vs entry.

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Scarlet.Communications in file Communications\Constants.cs:

• New enum type TypeID.

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Scarlet.Communications.Message in file Communications\Message.cs:

• Behavior change in method AppendData: now it will create payload if payload is null.
• Implementation change in method AppendData: use System.Buffer.BlockCopy to enhance efficiency.
• New public method GetDataSlice.

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Scarlet.Communications.Packet in file Communications\Packet.cs:

• New public method GetDataSlice.

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New file Communications\PacketScanner.cs:

• New class Scarlet.Communications.PacketScanner.

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New file Communications\PacketWriter.cs:

• New class Scarlet.Communications.PacketWriter.

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New file Communications\SafePacketScanner.cs:

• New class Scarlet.Communications.SafePacketScanner.

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New file Communications\SafePacketWriter.cs:

• New class Scarlet.Communications.SafePacketWriter.

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New unit test cases for new classes.