Improve deriving error messages

Instead of

    No instance for (Eq (Int -> Bool))
      arising from the 'deriving' clause of a data type declaration
    Possible fix:
      add an instance declaration for (Eq (Int -> Bool))
      or use a standalone 'deriving instance' declaration,
           so you can specify the instance context yourself
    When deriving the instance for (Eq Foo)

for

data Foo = Foo Int (Int -> Bool, Bool) deriving Eq

we want something like

    No instance for (Eq (Int -> Bool))
      arising from the second field of the constructor Foo
    Possible fix:
      add an instance declaration for (Eq (Int -> Bool))
      or use a standalone 'deriving instance' declaration,
           so you can specify the instance context yourself
    When deriving the instance for (Eq Foo)

where we tell the user more precisely from where in its data type the problem comes from. Should be possible by beefing up CtOrigin.

Trac metadata

Trac field	Value
Version
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler (Type checker)
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