Integrating with SB-SIMD

[phoe]

Since we now have packed arrays of structures, it should perhaps be possible to use SIMD to operate on these for going maximum brrrrrr. SB-SIMD became a contrib a while ago thanks to @marcoheisig; it provides an interface for SIMD instructions on SBCL. Will it be possible to somehow leverage it for use with unboxables?

One obstacle is that what we have here is not SIMD packs, or CL specialized vectors, but raw pointers with some external type information and possibly heterogenous data inside. Can SB-SIMD be somehow used with these?