Add support for associated type families

[jespercockx]

It would be useful and relatively easy to add support for associated types to type classes. Here's an example:

open import Haskell.Prelude

record Collection (c : Set) : Set₁ where
    field
        Elem : Set
        empty : c
        insert : Elem → c → c
        member : Elem → c → Bool
        toList : c → List Elem
open Collection {{...}} public

{-# COMPILE AGDA2HS Collection class #-}

instance
  iCollectionList : {{Eq a}} → Collection (List a)
  iCollectionList {a} = record
    { Elem   = a
    ; empty  = []
    ; insert = _∷_
    ; member = elem
    ; toList = id
    }

{-# COMPILE AGDA2HS iCollectionList #-}

Currently this produces a panic in agda2hs:

Panic: Pattern match failure in 'do' block at
src/Agda2Hs/Compile/Term.hs:147:7-26

[UlfNorell]

To think about: how to support the case when Elem is an associated data type?