Dict.fromList & Set.fromList duplicate key merge

[lue-bird]

implements #326 except for insert

Set.fromList [ a, a ] --> Set.fromList [ a ]
Dict.fromList [ ( a, v0 ), ( a, v1 ) ] --> Dict.fromList [ ( a, v1 ) ]

Honestly I'm surprised we didn't implement this already because it seems pretty useful.

The check for duplicates is n^2 but I guess that shouldn't be too bad because these explicit fromList calls are rare and usually not terribly long in non-codegen code.

[jfmengels]

Later improvement: I guess we could do this check but only for literals if expectNaN is True.

[jfmengels]

This looks great, thanks @lue-bird!