Move FastByteReader from LCW to own file

And make it public.
Making it easier to move formats that depend on it
to Mods.Cnc independently later.

OpenRA.Mods.Common/FileFormats/FastByteReader.cs

@@ -0,0 +1,43 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2019 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version. For more
+ * information, see COPYING.
+ */
+#endregion
+
+using System;
+
+namespace OpenRA.Mods.Common.FileFormats
+{
+	public class FastByteReader
+	{
+		readonly byte[] src;
+		int offset;
+
+		public FastByteReader(byte[] src, int offset = 0)
+		{
+			this.src = src;
+			this.offset = offset;
+		}
+
+		public bool Done() { return offset >= src.Length; }
+		public byte ReadByte() { return src[offset++]; }
+		public int ReadWord()
+		{
+			var x = ReadByte();
+			return x | (ReadByte() << 8);
+		}
+
+		public void CopyTo(byte[] dest, int offset, int count)
+		{
+			Array.Copy(src, this.offset, dest, offset, count);
+			this.offset += count;
+		}
+
+		public int Remaining() { return src.Length - offset; }
+	}
+}