block expects on opaque types

[rvcas]

• added a new field called opaque: bool to Type::App (can be used to simplify some things in code gen too)
• implement a simple method on Type called is_opaque that leverages the new field
• make a new error for when someone uses expect with an opaque type in anyway
• catch this situation in infer_assignment and return the new error

I figured that opaque does not need to affect the type equality

[Quantumplation]

Perhaps the error message can briefly explain what an opaque type is, and why this isn't allowed? (for example, I don't know the answer to either of those questions heh)

[KtorZ]

[Quantumplation]

:+1: on the error message <3

[t0anh]

Seems this causes unexpected effects. How should I deal with datums consisting Rational or Dict?

[KtorZ]

@t0anh These aren't unexpected effects, these are precisely the expected effects!

For Dict, you should pass an ordered list of key/values as an ordered list, and then use dict.from_ascending_list to turn it into a list.

For Rational, you should pass the numerator and denominator and then use rational.new to convert it back into a rational.