Migrate recent HLLib work to BOS

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

BurnOutSharp.Models/GCF/BlockEntryMap.cs

@@ -0,0 +1,19 @@
+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 BlockEntryMap
+    {
+        /// <summary>
+        /// The previous block entry.  (N/A if == BlockCount.)
+        /// </summary>
+        public uint PreviousBlockEntryIndex;
+
+        /// <summary>
+        /// The next block entry.  (N/A if == BlockCount.)
+        /// </summary>
+        public uint NextBlockEntryIndex;
+    }
+}