Fix tuple types

[chuwy]

I just noticed that patterns like (Nat, Nat) produce an error.

I'm not 100% sure it's a good idea to combine (Optional Nat) and (Nat, Nat) as it feels lossy, but it repeats the pattern we have in terms.

[kylegoetz]

Thanks! I'll write a couple regression tests and likely merge.

I forget in this specific scenario why we have the "tuple_or_parenthetical" but generally it has to do with it being impossible from just parsing to tell the difference between

(1-tuple) and (1-item parenthetical)

To tell the difference, you need access to types that a simple parser can't give you.

[kylegoetz]

LGTM!