Create fastfilter() stream method.

[asuiu]

Use already implemented fastmap()

[asuiu]

As a simpler implementation, it can use fastmap behind, in this way:

def fastmap(self, f:Callable)
    return (self.fastmap(lambda _: (_, f(_)) )
        .filter(itemgetter(1))
        .map(itemgetter(0)
        )