Fix loading specialized vectors from fasls.

Previously vectors were dumped and loaded based on the hand crafted
typecase forms, which was quite fragile.
After the dumping part was rewritten to consult
sb-vm:*specialized-array-element-type-properties*, the loading part
broke down.
Change the way it's done altogether, simplifying things significantly.
Instead of having separate FOPs for signed and unsigned vectors,
writing bit size of elements, just dump the widetag and use just a
single FOP, fop-spec-vector. Floating point dumping routines now use
fop-spec-vector too.