Migrate recent HLLib work to BOS

It's better suited for the model of BOS and not an update to HLLibSharp

BurnOutSharp.Models/GCF/BlockEntryMapHeader.cs

@@ -0,0 +1,34 @@
+namespace BurnOutSharp.Models.GCF
+{
+    /// <remarks>
+    /// Part of version 5 but not version 6.
+    /// </remarks>
+    /// <see href="https://github.com/RavuAlHemio/hllib/blob/master/HLLib/GCFFile.h"/>
+    public sealed class BlockEntryMapHeader
+    {
+        /// <summary>
+        /// Number of data blocks.
+        /// </summary>
+        public uint BlockCount;
+
+        /// <summary>
+        /// Index of the first block entry.
+        /// </summary>
+        public uint FirstBlockEntryIndex;
+
+        /// <summary>
+        /// Index of the last block entry.
+        /// </summary>
+        public uint LastBlockEntryIndex;
+
+        /// <summary>
+        /// Reserved
+        /// </summary>
+        public uint Dummy0;
+
+        /// <summary>
+        /// Header checksum.
+        /// </summary>
+        public uint Checksum;
+    }
+}