A delightful mutant hybrid of two seemingly incongruous teen movie genres, cheeky Blumhouse satire “Freaky” might just as well have been called “Freaky Friday the 13th.” That’s essentially the pitch for a mashup that’s half slasher movie and half body-swap comedy, delivered with the self-aware wink of Wes Craven’s “Scream,” as a serial killer known as “the Blissfield Butcher” (Vince Vaughn) unwittingly trades places with his latest victim, dorky high school misfit Millie (Kathryn Newton).