Fix how Sets and Dicts are shown

[jvoigtlaender]

Addresses the Set-related part of https://github.com/elm-lang/core/issues/288.

Before this PR, using toString/show on the following three expressions:

• Set.fromList [1,2,3]
• Dict.fromList [(1,()),(2,()),(3,())]
• Dict.fromList [(1,1),(2,2),(3,3)]

would lead to:

• "Set (Set.fromList [1,2,3])"
• "Set.fromList [1,2,3]"
• "Dict.fromList [(1,1),(2,2),(3,3)]"

After this PR, the resulting strings are:

• "Set.fromList [1,2,3]"
• "Dict.fromList [(1,()),(2,()),(3,())]"
• "Dict.fromList [(1,1),(2,2),(3,3)]"

[evancz]

Alright, looks good to me, thank you!

[evancz]

Alright, looks good to me, thank you!