ModelStream should be a drop-in replacement for Model or their API differences should be documented

[hudson-ai]

ModelStreams (returned from Model.stream()) seem to be drop-in replacements for Model objects, at least when it comes to appending grammars. But they do not support:

• __getitem__
• __str__
• role blocks (fail silently, similar to Role blocks fail silently inside of stateless grammars #916)

They should either be brought in line with ModelStream being a drop-in replacement, or additional documentation about the cases where this is not the case (and maybe warnings/exceptions should be raised?)

[peterwilli]

For the role blocks, I made a temporary workaround at #943 (I didn't see this issue was mentioned before, sorry!)