Add parFmap function

[Boarders]

Control.Parallel.Strategies defines:

parMap :: Strategy b -> (a -> b) -> [a] -> [b]
parMap strat f = (`using` parList strat) . map f

However parList is defined in terms of parTraversable:

parList :: Strategy a -> Strategy [a]
parList = parTraversable

This pull request generalises parMap to:

parFmap :: Traversable t => Strategy b -> (a -> b) -> t b -> t b
parFmap strat g = (`using` parTraversable strat) . fmap f

The old definition is still left in place.