Export SB-SIMD-PACK symbols from SB-EXT

 Export the SIMD-PACK type, the SIMD-PACK-P predicate,
 %make-simd-pack-{integer,single,double}, and
 %simd-pack-{ub64s,singles,doubles}.

 These are far from useful yet, but at least future extensions
 can work with SB-EXT instead of SB-KERNEL.

 Also, says so in NEWS.