New function `mapOr : (a -> b) -> b -> Maybe a -> Maybe b`

[Aleod-m]

I find this pattern often in various cases where i want to map an optional value or put a default if its nothing but keep it optional.

mapOr : (a -> b) -> b -> Maybe a -> Maybe b
mapOr fn default ma =
	case ma of 
		Nothing -> Just default 
		Just a -> Just (fn a)
-- Changes :
 ma |> Maybe.map f |> Maybe.withDefault d |> Just 
-- or
 ma |> Maybe.map f |> Maybe.Extra.orElse (Just d)
-- To :
 ma |> MapOr f d