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

~4x wall-clock improvement on my laptop.

psuriana/Halide,kgnk/Halide,kgnk/Halide,psuriana/Halide,psuriana/Halide,psuriana/Halide,kgnk/Halide,psuriana/Halide,kgnk/Halide,kgnk/Halide,psuriana/Halide,kgnk/Halide,kgnk/Halide,kgnk/Halide,psuriana/Halide