Nicknaming API for improved debugging

[dphfox]

#323 introduces an internal nicknames table which is used to annotate various values with human-readable descriptions for the purposes of improved debugging. For example, objects can label the tasks they add to scopes, so that error messages can understandably report which objects are involved in the message in a way that allows developers to rediscover the root cause.

There could be value in exposing a function that allows public users to set nicknames. It's unclear whether this should be part of debugging protocol as in #306 or a standalone API that doesn't require debugging tools.