Prevent copying folders inside themselves

- contains now performs a case-insensitive comparison. While this is a
   little dubious, it's required for systems like Windows, where foo and
   Foo are the same folder.
 - Impose a depth limit on copyRecursive. If there are any other cases
   where we may try to copy a folder into itself, this should prevent
   the computer entirely crashing.

src/main/java/dan200/computercraft/api/filesystem/FileOperationException.java

@@ -27,7 +27,7 @@ public class FileOperationException extends IOException
         this.filename = filename;
     }
 
-    public FileOperationException( String message )
+    public FileOperationException( @Nonnull String message )
     {
         super( Objects.requireNonNull( message, "message cannot be null" ) );
         this.filename = null;