Mark ClassesC as Trustworthy

[langston-barrett]

For compatibility with lens < 5. Fixes #152.

[RyanGlScott]

GHC now warns:

[20 of 38] Compiling Data.Parameterized.ClassesC
src/Data/Parameterized/ClassesC.hs:20:14: warning: [-Wtrustworthy-safe]
Warning:     ‘Data.Parameterized.ClassesC’ is marked as Trustworthy but has been inferred as safe!
   |
20 | {-# LANGUAGE Trustworthy #-}
   |              ^^^^^^^^^^^

I think we should just globally set -Wno-trustworthy-safe in the .cabal file to avoid this class of warnings.

[langston-barrett]

Good catch, fixed!

[RyanGlScott]

I think this is a sensible way to handle things. Does this work for you, @kquick?

[kquick]

Couldn't subterms someone something internally call unsafePerformIO and wouldn't this violate the intention of declaring this module Trustworthy?

[RyanGlScott]

My understanding of the contract that Trustworthy offers (as documented here) is that Trustworthy code is allowed to do unsafe things internally, provided that the user-facing API still respects type safety and referential transparency. That is certainly the case with Data.Parameterized.ClassesC, and I'd wager the same applies to nearly all the code in parameterized-utils (with the possible exception of Data.Parameterized.Axiom).

[kquick]

OK. I have reservations about who bears the responsibility on Trustworthy, but I think that's outside the scope of what is reasonably involved here, so I'm good with this change.

[RyanGlScott]

Are you alright with merging this, @langston-barrett?