Fixed Credential using a fixed-size 64-bit handle; it's always 2 pointers.

Reworked the Credential handle into a SafeCredentialHandle. We still use references to the raw handle, which is unsafe, but I will hopefully rework that soon.

NativeMethods.cs

@@ -58,7 +58,7 @@ namespace NSspi
             IntPtr packageData,
             IntPtr getKeyFunc,
             IntPtr getKeyData,
-            ref long credentialHandle,
+            ref RawSspiHandle credentialHandle,
             ref long expiry
         );
 
@@ -75,7 +75,7 @@ namespace NSspi
             SetLastError = true
         )]
         public static extern SecurityStatus FreeCredentialsHandle(
-            ref long credentialHandle
+            ref RawSspiHandle credentialHandle
         );
 
         /*
@@ -118,7 +118,7 @@ namespace NSspi
             SetLastError = true
         )]
         public static extern SecurityStatus QueryCredentialsAttribute_Name(
-            ref long credentialHandle,
+            ref RawSspiHandle credentialHandle,
             CredentialQueryAttrib attributeName,
             ref QueryNameAttribCarrier name
         );