problems with frozen dataclass and hashable requirements of Meaning

[nathimel]

We need to either represent the prior not as a dict, or do something different with the dataclass implementation, because the the dict is not hashable.

Grammar enumeration, but probably other things too, expect a Meaning to be hashable. But as a frozen dataclass this means we can't use a dict to represent the prior over referents. E.g.
https://github.com/CLMBRs/ultk/blob/main/src/ultk/language/grammar.py#L316
results in
*** TypeError: unhashable type: 'dict'

The hashability of the list of referents is related to this. We assumed it could be an iterable, and our tests pass a list, but a list is not hashable. #26

Happy to represent the prior as simply a tuple, since I've proposed requiring referents to be a tuple. So recovering order for the mapping won't be a problem, and we can still have a function that returns the dict.

[nathimel]

And this means that Universe will also have to change since it assumes a prior