grammar enumeration requires Meaning to be hashable, but we allow referents to be a list

[nathimel]

Small thing that I will fix myself, but documenting here anyways. In general I feel like we should have more guardrails against this kind of thing in the grammar enumeration code.

Nowhere in Meaning is it required to pass referents as a tuple, and it is not converted to one:
https://github.com/CLMBRs/ultk/blob/main/src/ultk/language/semantics.py#L133

But in grammar enumeration, and probably elsewhere, we assume Meaning is hashable. Since Meaning is now a dataclass, we should require referents to be a tuple.

https://github.com/CLMBRs/ultk/blob/main/src/ultk/language/grammar.py#L316

[nathimel]

My first pass is going to be just to raise an error if anything other than a tuple of referents is passed