Add Iterator to loop over vector using VecRange

[Zentrik]

using SIMD
function vadd!(xs::Vector{T}, ys::Vector{T}, ::Type{Vec{N,T}}) where {N, T}
    @assert length(ys) == length(xs)
    @assert length(xs) % N == 0

    @inbounds for lane in LoopVecRange{N}(xs)
        xs[lane] += ys[lane]
    end
end

This avoids the current performance issue with using StepRange (fixed now from Julia v1.10) and is a simpler way to loop.

Not sure if I should have has_offset_axes, I just copied it from StepRange.
Also not sure if this is the right use for @boundscheck. I added unsafe as @inbounds wasn't working with the masked tests.

[Zentrik]

The performance issue with the existing approach (i.e. using a StepRange) should be solved on Julia master.
This still strikes me as a nice to have though.

[MasonProtter]

Bump @KristofferC @eschnett