Performance improvements and 0.7 compatibility
More 0.7 compat (#52) * `copy!()` -> `copyto!()` * Quash more 0.7 compatibility bugs * Require at least Compat 0.38
More 0.7 compat (#52) * `copy!()` -> `copyto!()` * Quash more 0.7 compatibility bugs * Require at least Compat 0.38