Using SIMD types for vector addition

[ksluder (Kyle Sluder)]

@wigging

func add(_ a: [Double], _ b: [Double], result: inout [Double]) {
    for i in a.indices {
        result[i] = a[i] + b[i]
    }
}

The Swift version is doing overflow checking. What happens if you change a[i] + b[i] to a[i] &+ b[i]?