In the fantastical Finnish horror fairytale “Hatching,” the directorial debut of Hannah Bergholm, a young girl hatches a murderous bird monster out of an egg that she secretly nests in her bed, and that’s not even the scariest part — her perfectionist mommy blogger mother strikes the truest terror in the film. Like many great monster movies, “Hatching” uses a monster as a metaphor for repressed emotion, and the creature at the center of this film is one of the most uniquely grotesque creations seen on screen in a long time. The bird monster is the creation of Gustav Hoegen, a renowned animatro…