Add function keys and annotate ratings

[cthoyt]

Summary

This PR streamlines generating metric sets and annotates more functions.

• [x] Code passes all tests
• [x] Unit tests provided for these changes
• [x] Documentation and docstrings added for these changes

Changes

• [x] Update the rexmex.utils.Annotator class to include a key. If it's not given, this defaults to the function's name. The registry now uses the key instead of the function's name
• [x] Update the metric sets to load the function names directly from the keys in the annotator's dictionary
• [x] Annotate functions in the ratings module
• [x] Generalizes tests to make it easier to test the existence of annotations for ratings, coverage, and rankings