better SB-INT:MIX

Use a large prime multiplier rather than 3 in the linear mix.  Potentially
slower, particularly on x86 without :pentium4 where it might get implemented
as a large bunch of LEAs, but better output.
improve SXHASH on fixnums

Remove one redundant LOGAND; also, use the whole of the fixnum
range on 64-bit platforms.
alter ftype declarations in genesis.lisp

change (or descriptor symbol) to (or symbol descriptor) to work
around a CCL bug regarding type test elision.