Add splat_lane

[calebzulawski]

Like splat, but from an existing vector.

[thomcc]

I'm not sure this makes that much sense as a public method (although I'm not sure it's not worth having, it just seems a little specific of a shuffle), but I do think it's worth using inside splat if it's needed to fix https://rust-lang.zulipchat.com/#narrow/stream/257879-project-portable-simd/topic/Very.20bad.20.60u16x8.3A.3Asplat.60.20codegen.20on.20x86_64

[programmerjake]

idk that we should have splat_lane as a public API, considering how easy it is to do:

pub fn splat_lane_0(v: u16x8) -> u16x8
{
    simd_swizzle!(v, [0; 8])
}

other than that, lgtm.

[calebzulawski]

I suppose it is pretty trivial with the macro, agreed. Trivial enough to add in the future if requested