`fromList` fold/overwrite behaviour toggle

[ocramz]

Generalize fromListSV/fromListSM to accept a parameter to either overwrite or apply some fold over repeated elements.
The current behaviour is overwriting.

Example of new behaviour:

fromListSV (FoldRepeated (+) 0) [(1, 1), (2, 3), (2, 5)] == fromListSV OverwriteRepeated [(1, 1), (2, 8)]