Speed up vector_cast test by using std::async
Speed up vector_cast test by using std::async

~4x wall-clock improvement on my laptop.


Former-commit-id: 2772afddd3fd487337b7c2e419198692f9a7ec74
Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide