User output should not show eta-contracted data instances

Consider

data family Sing (a :: k)
data instance Sing (a :: [k]) = SNil
data instance Sing Bool = SBool

If you load this into ghci and say :info Sing you get

:i Sing
type role Sing nominal
data family Sing (a :: k)
  	-- Defined at T8557.hs:4:1
data instance Sing Bool -- Defined at T8557.hs:6:15
data instance Sing -- Defined at T8557.hs:5:15

The data instance is eta-contracted (see Note [Eta reduction for data family axioms] in TcInstDcls).

This is jolly confusing for our users. We should eta-expand before displaying.

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture