Transform/extension methods

New phase ported: extension methods
For now we have two methods of reporting error. 
Most methods in failure return a `NoDenotation` or `NoSymbol`. But some throw an exception, like this. 
For now one needs to remember which methods use which type of error reporting. 
Maybe it would be more uniform to make `owner` return `NoSymbol` for `NoDenotation` and remove need for `maybeOwner`?