Migrate recent HLLib work to BOS

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

BurnOutSharp.Models/GCF/FragmentationMapHeader.cs

@@ -0,0 +1,26 @@
+namespace BurnOutSharp.Models.GCF
+{
+    /// <see href="https://github.com/RavuAlHemio/hllib/blob/master/HLLib/GCFFile.h"/>
+    public sealed class FragmentationMapHeader
+    {
+        /// <summary>
+        /// Number of data blocks.
+        /// </summary>
+        public uint BlockCount;
+
+        /// <summary>
+        /// The index of the first unused fragmentation map entry.
+        /// </summary>
+        public uint FirstUnusedEntry;
+
+        /// <summary>
+        /// The block entry terminator; 0 = 0x0000ffff or 1 = 0xffffffff.
+        /// </summary>
+        public uint Terminator;
+
+        /// <summary>
+        /// Header checksum.
+        /// </summary>
+        public uint Checksum;
+    }
+}