Implement support for parsing/interacting with NCAs. (#942)

* fs: implement support for interacting with ncas.

* spl: extend to use virtual keyslots

stratosphere/spl/source/spl_fs_service.cpp

@@ -35,7 +35,7 @@ namespace ams::spl {
         return impl::GenerateSpecificAesKey(out_key.GetPointer(), key_source, generation, which);
     }
 
-    Result FsService::LoadTitleKey(u32 keyslot, AccessKey access_key) {
+    Result FsService::LoadTitleKey(s32 keyslot, AccessKey access_key) {
         return impl::LoadTitleKey(keyslot, this, access_key);
     }