Jessica Kiang Splicing the dark heart of a folk-horror movie into the fluffy body of a rural Icelandic relationship drama yields unexpectedly fertile and darkly comic effects in Valdimir Jóhannsson’s creepy-funny-weird-sad “Lamb,” a film that proves just how far disbelief can be suspended if you’re in the hands of a director — and a cast, and an SFX/puppetry department — who really commit to the bit.