NormFile Refactor

* Updated NormFileGetName to use sbyte* to avoid casting

* Updated design

src/dotnet/design/Mil/Navy/Nrl/Norm/NormApi.puml

@@ -97,7 +97,7 @@ class NormApi <<static>> {
     + {static} <<extern>> NormObjectCancel(objectHandle:long) : void
     + {static} <<extern>> NormObjectRetain(objectHandle:long) : void
     + {static} <<extern>> NormObjectRelease(objectHandle:long) : void
-    + {static} <<extern>> NormFileGetName(fileHandle:long, nameBuffer:nint, bufferLen:int) : bool
+    + <<unsafe>> {static} <<extern>> NormFileGetName(fileHandle:long, nameBuffer:sbyte*, bufferLen:int) : bool
     + {static} <<extern>> NormFileRename(fileHandle:long, fileName:string) : bool
     + {static} <<extern>> NormDataAccessData(objectHandle:long) : nint
     + {static} <<extern>> NormObjectGetSender(objectHandle:long) : long

src/dotnet/src/Mil.Navy.Nrl.Norm/NormApi.cs

@@ -1020,7 +1020,7 @@ namespace Mil.Navy.Nrl.Norm
         /// does not refer to an object of type NORM_OBJECT_FILE.
         /// </returns>
         [DllImport(NORM_LIBRARY)]
-        public static extern bool NormFileGetName(long fileHandle, nint nameBuffer, int bufferLen);
+        public unsafe static extern bool NormFileGetName(long fileHandle, sbyte* nameBuffer, int bufferLen);
 
         /// <summary>
         /// This function renames the file used to store content for the NORM_OBJECT_FILE transport object specified by 

src/dotnet/src/Mil.Navy.Nrl.Norm/NormFile.cs

@@ -1,7 +1,4 @@
-using System.Runtime.InteropServices;
-using System.Text;
-
-namespace Mil.Navy.Nrl.Norm
+namespace Mil.Navy.Nrl.Norm
 {
     /// <summary>
     /// A transport object of type NORM_OBJECT_FILE.
@@ -29,16 +26,14 @@ namespace Mil.Navy.Nrl.Norm
         {
             get
             {
-                var buffer = stackalloc byte[FILENAME_MAX];
-                var bufferPtr = (nint)buffer;
+                var buffer = stackalloc sbyte[FILENAME_MAX];
 
-                if (!NormFileGetName(_handle, bufferPtr, FILENAME_MAX))
+                if (!NormFileGetName(_handle, buffer, FILENAME_MAX))
                 {
                     throw new IOException("Failed to get file name");
                 }
-                var name = Marshal.PtrToStringAnsi(bufferPtr);
                 
-                return name ?? string.Empty;
+                return new string(buffer);
             }
         }