Support Document Models with dicts in LLMRater

[pl04351820]

This PR fixes a TypeError in llmrater.py that occurred when processing document models (nested lists/dicts).

Changes:

• util.make_hashable: Fixed to recursively hash list items so nested dictionaries are properly converted into hashable frozensets.
• setmatcher.py: Cleaned up the redundant inline _make_hashable from PR Generalize SetMatcher to support Document Model #288 to use util.make_hashable. Also migrated multiset equality to use collections.Counter instead of sorted(), which is unstable for frozenset objects in Python 3.
• llmrater_test.py: Added a new unit test to verify that take_n_uniques safely handles nested document structures.

[IsmailMehdi]

/gcbrun