Additional niceties and middle end support for short vector SIMD packs

Additional niceties and middle end support for short vector SIMD packs

 * Allow FASL loading/dumping of (boxed) SIMD packs, and mark them as
   trivially (i.e. without going through make-load-form) dumpable.

 * SIMD packs print nicely, and take the element type into account while
   doing so.

 * (C)TYPE-OF is more accurate for SIMD packs; this enables IR2 conversion
   to choose the right primitive type and storage class for constants.

The FASL code was kept on life support by Alexander Gavrilov for too many years,
and the printing logic is a very light adaptation of the output code he developed
for his branch.