Allow `existing-class` for postulated type classes

[flupe]

We should enable the existing-class pragma for postulated type classes:

postulate
  MyShow : Set -> Set
  myshow : {{ MyShow a }} -> a -> String
{-# COMPILE MyShow existing-class #-}

ok : {{ MyShow a }} -> a -> String
ok = myshow

Currently MyShow is not considered a class (since #284), and thus the instance arguments get turned into regular Haskell arguments and not class constraints:

ok : MyShow a -> a -> String
ok x = myshow x

[jespercockx]

@flupe It seems you worked on this issue, do you have a fix that is ready for the release of agda2hs 1.4 (hopefully soon)?

[flupe]

Hm, I implemented the feature in #374, but ended up closing the PR as, quoting myself:

I can't think of any situations where it isn't preferable to introduce a record [for the Class interface] and use existing-class [on it].

So I guess we can actually close this issue (or merge #374, as you prefer).

[jespercockx]

Ok, I don't see a strong reason for including this either so let's close this issue.