Write the function merge_dicts(L) that takes in a list of dictionaries and non-destructively merges them.

For example if L = [{"a": 1, "b": 2, "c": 3}, {"a": 4, "b": 2, "c": 3}], then merge_dicts(L) should return {"a": {1, 4}, "b": 2, "c": 3}